(CNN)George Floyd's death has raised new questions about a potentially dangerous method of restraint: the knee-to-neck maneuver.
Floyd, a 46-year-old black man, was held down with a knee as he protested that he couldn't breathe. The FBI is investigating the incident, which drew widespread condemnation of the officers, who were fired this week, after a video showing part of the encounter circulated on social media.
The knee-to-neck move is banned by several major metropolitan police departments, but Minneapolis police allow police to restrain suspects' necks if they're aggressive or resisting arrest. Floyd was unarmed and handcuffed when he was pinned to the ground.
Some law enforcement experts say the move is dangerous and unnecessary. Here's why.
It could cause injury
Depending on the way someone's head is positioned and the weight of the person applying pressure to their neck, the knee-to-neck maneuver can cause significant damage, said Seth Stoughton, an associate professor of law at the University of South Carolina.
Stoughton, co-author of "Evaluating Police Uses of Force," identified three ways officers can cause potential injury using the knee-to-neck method.
The first, he said, is keeping suspects in the prone position, meaning they lie face-down with their hands cuffed behind their backs, for an extended period of time. It's a dangerous position, Stoughton said, because it's known to cause what's called positional asphyxia.
Someone in that position can draw enough breath to gasp or speak in spurts, but they can't breathe fully, so they gradually lose oxygen and fall unconscious.
George Floyd's brother called him a "gentle giant" who moved to Minneapolis to improve his life.
Stoughton, who is a former officer, said police departments emphasize keeping someone in the prone position just long enough to restrain them, then take them out of it either by rolling them onto their side, sitting them up or having them stand.
Another potential injury involves the placement of pressure. A great deal of pressure and weight on a suspect's neck can cause fatal damage.
In his book on use of force, Stoughton wrote, "Officers should avoid putting their body weight on the subject's neck or head; the pressure of such a position can fracture the hyoid bone or cervical spine, depending on the position of the subject's head."
Finally, any officers involved in this maneuver must closely monitor the suspect's well-being, he said. Kneeling on their neck for an extended period of time could be fatal.
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Protesters march in the street during a demonstration in Minneapolis on June 25.
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People in New York City paint a Black Lives Matter mural on the street directly outside of Trump Tower on Thursday, July 9.
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Willie Townsend, an employee of the Mississippi State Capitol, raises the state flag for its retirement ceremony on July 1. Gov. Tate Reeves signed a bill to retire the flag — the last state flag to feature the Confederate battle flag. The flag of the Confederacy and its symbols have long divided the country. Critics call the flag a symbol that represents the war to uphold slavery, while supporters call it a sign of Southern pride and heritage.
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People dance in New York City during a protest to defund the police on June 26.
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Fireworks explode over the statue of Confederate General Robert E. Lee during a Juneteenth celebration in Richmond, Virginia, on June 19. The Juneteenth holiday commemorates the end of slavery in the United States.
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A man kneels and raises his fist in the air at the Martin Luther King Jr. Memorial in Washington, DC, on June 19.
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A statue of Confederate Maj. Richard W. Dowling is removed in Houston on June 17. Confederate statues are being taken down and tampered with across the United States.
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Members of Spark-Y, a nonprofit youth empowerment group, build a garden at the George Floyd memorial site in Minneapolis on June 17.
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A woman shouts slogans as she protests outside a burned Wendy's restaurant in Atlanta on June 15. Demonstrators set fire to the restaurant on June 13 after Rayshard Brooks was fatally shot near the restaurant's drive-thru on June 12. Brooks, 27, was shot after police moved to handcuff him for suspected driving under the influence, according to videos from the scene. The videos show that Brooks took an officer's Taser during the attempted arrest and then fired the Taser at the officers as he ran away. One officer then fatally shot Brooks three times with his service weapon, authorities said. Brooks was shot twice in the back, according to a release by the Fulton County Medical Examiner's Office.
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Police in Albuquerque, New Mexico, detain members of the New Mexico Civil Guard, an armed civilian group, after a man was shot during a protest on June 15. The shooting happened as protesters were trying to pull down a statue of Spanish conquistador Juan de Oñate. A 31-year-old man was arrested in connection with the shooting, police said. The New Mexico Civil Guard told CNN by email that the man is not part of their group.
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Protesters surround the police headquarters in Richmond, Virginia, on June 14.
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This aerial photo shows a Black Lives Matter mural in Seattle on June 14. Barricades and street graffiti mark the entrance to what's known as Seattle's Capitol Hill Autonomous Zone, which protesters have occupied since June 7.
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Demonstrators set fire to a Wendy's restaurant in Atlanta on June 13. Rayshard Brooks was fatally shot by police on Friday night.
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A police officer talks to protesters on an Atlanta highway, near where Brooks was killed. A major interstate was shut down after protesters marched onto a connector and were met by lined-up police vehicles.
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Protesters rally in Atlanta after Brooks' death.
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Demonstrators in New York make their way through Brooklyn on June 13.
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Paperboy Prince, a congressional candidate for New York's 7th District, leads a march in Brooklyn on June 12.
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Members of the 1199SEIU union, the nation's largest union of health-care workers, kneel for eight minutes and 46 seconds during a vigil at the Brookdale Hospital Medical Center in New York. Eight minutes and 46 seconds is how long the police officer in Minneapolis held his knee on George Floyd's neck.
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Minnesota state troopers surround a statue of Christopher Columbus after activists pulled it down in front of the Capitol in St. Paul on June 10. Columbus has long been a contentious figure in history for his treatment of the Indigenous communities he encountered and for his role in the violent colonization at their expense.
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Josiah Brown fist-bumps a member of the US Secret Service after he and his mother, Alexis Brown, prayed over the officers near the White House on June 9.
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Buffalo Bills cornerback Josh Norman prays over Malcolm Rutledge during protests near the White House on June 9. "Both White and Black people haven't seen a Black man in chains in years," Rutledge said. "They don't understand the psychological chains we still carry."
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New York City Public Advocate Jumaane Williams, center, leads a march over the Brooklyn Bridge during a protest for police reform on June 8.
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Black Lives Matter supporters gather in Los Angeles for a memorial service honoring George Floyd on June 8.
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Seattle Police and Washington National Guard personnel retake control of an intersection as demonstrators clash with law enforcement near the Seattle Police Department's East Precinct shortly after midnight on June 8.
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A person watches protesters march in St. Louis on June 7.
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A young boy rides with the Compton Cowboys during a "peace ride" for George Floyd in Compton, California, on June 7.
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Protesters lie in a street near the White House on June 7.
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A man dances near Floyd's memorial in Minneapolis on June 6.
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Demonstrators paint the words "defund the police" as they protest near the White House on June 6.
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People protest near the Seattle Police Department's East Precinct on June 6.
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Protesters stretch more than five blocks during a demonstration near the White House on June 6.
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Xavier Brown shows his support as demonstrators march past his home in St. Paul, Minnesota, on June 6.
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Protesters walk across the Brooklyn Bridge on June 6.
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Tamika Palmer, the mother of Breonna Taylor, closes her eyes during a vigil for her daughter in Louisville, Kentucky, on June 6.
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Demonstrators take to the streets in Clayton, Missouri, on June 6.
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Protesters are arrested after violating curfew in New York on June 5.
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People attend a rally outside Jackson Square in New Orleans on June 5.
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The new Black Lives Matter Plaza is seen near St. John's Episcopal Church in Washington, DC, on June 5. The words "Black Lives Matter" were painted on two blocks of 16th Street. The painters were contracted by Mayor Muriel Bowser.
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Demonstrators gather in St. Paul, Minnesota, on June 5.
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Demonstrators brave the rain to protest near the White House on June 4.
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Protesters take a knee in front of a line of police officers in Brooklyn, New York, on June 4.
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Protesters march in San Diego on June 4.
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Police in New York arrest a protester on Fifth Avenue on June 4.
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The White House is seen behind temporary metal fencing that was installed to keep protesters further back.
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Jessica Moore attempts to hold dialogue with a counter-protester while rallying in Anna, Illinois, on June 4.
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People dance in the street during a protest in Pasadena, California, on June 4.
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Protesters in Washington, DC, gather at the Martin Luther King Jr. Memorial on June 4.
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A man in Minneapolis holds a candle June 3 near the scene of George Floyd's death.
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Protesters use umbrellas during a protest in Seattle on June 3.
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Protesters in New Orleans gather in front of a pedestal that once displayed a statue of Confederate soldier Robert E. Lee. The Civil War-era landmark was removed in 2017 after a nationwide debate over Confederate symbols.
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Protesters hold up their phones during a demonstration outside the White House on June 3.
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A protester is arrested for violating curfew near the Plaza Hotel in New York on June 3.
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Hundreds of surfers in Encinitas, California, gather in support of Black Lives Matter on June 3.
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A protester faces a law enforcement officer in Washington, DC, on June 3.
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Protesters pack into Dolores Park during a demonstration in San Francisco on June 3.
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Protesters kneel in front of New York City police officers before being arrested for violating curfew on June 3.
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Hundreds of demonstrators in Boston lie face down, symbolizing the last moments of George Floyd's life, on June 3. Related photos: Lie-in protests around the world
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People protest near the US Capitol in Washington, DC, on June 3.
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Atlanta Police Officer J. Coleman shares a fist bump with protester Elijah Raffington on June 3. The police bicycle unit was taking a knee with protesters outside the CNN Center.
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Demonstrators march through Brooklyn, New York, on June 3.
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People in Minneapolis raise their hands and shout slogans on June 2 as they protest at the makeshift memorial for George Floyd.
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Protesters walk off the Manhattan Bridge in New York after being blocked by police on June 2. Police were on both sides of the bridge as peaceful protesters were in the middle. Eventually the protesters were allowed to walk away and leave the area.
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Protesters lie down in an intersection, blocking traffic in Coralville, Iowa, on June 2.
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Police officers hold a perimeter June 2 behind a metal fence that was recently erected in front of the White House.
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Protesters ride a bus through a street in Atlanta on June 2. The windshield reads "use your voice."
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Demonstrators raise their hands in the air while protesting in front of City Hall in Los Angeles on June 2.
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Protesters gather in Houston on June 2. Tens of thousands of people marched to City Hall to shout George Floyd's name. Houston is Floyd's hometown.
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Protesters raise their fists in New York City on June 2.
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Members of the National Guard watch as demonstrators march along Hollywood Boulevard in Los Angeles on June 2.
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Protesters on horseback rally in downtown Houston on June 2.
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A group of clergy members stops and prays during a march to a George Floyd memorial in Minneapolis on June 2.
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Ericka Ward-Audena stands with her 7-year-old daughter, Elle, during a protest in Washington, DC, on June 2. "I wanted my daughter to see the protests," she said. "It's really important. I've gotten a million questions from her because of it."
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Protesters gather near Manhattan's Foley Square in New York on June 2.
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Protesters rally in Bethesda, Maryland, on June 2.
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A resident of Clarksville, Tennessee, holds up a sign that says "I can't breathe" across the street from protesters on June 2.
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Protesters rally outside the Ohio Statehouse in Columbus on June 2.
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Passengers hold up their fists in solidarity with protesters as they drive by the Wood County Courthouse in Wisconsin Rapids, Wisconsin, on June 2.
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A person holds a "Black Lives Matter" sign as a heavy cloud of tear gas and smoke rises in Seattle on June 1.
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People attend a candlelight vigil at Queens Park in New York on June 1.
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Two men kneel in front of a line of Kentucky state troopers during a protest in Louisville on June 1.
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A firework thrown by a protester explodes at the feet of police in Riverside, California, on June 1.
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A woman cries out after being exposed to tear gas near the White House on June 1. Thousands of people were peacefully protesting near Lafayette Park when police started to shoot rubber bullets, tear gas and flash bangs into the crowd. They were clearing the block to allow President Donald Trump to walk to St. John's Episcopal Church for a photo op.
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Security forces push protesters away from the White House in order to allow President Trump to make a visit to a nearby church on June 1.
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Law enforcement officers kneel with protesters in Atlanta on June 1.
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A protester is doused with water and milk after being hit with pepper spray from law enforcement in Washington, DC, on June 1.
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Protesters gather at the J.E.B. Stuart statue in Richmond, Virginia, on June 1.
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A demonstrator holds her hands up while she kneels in front of police officers at City Hall in Anaheim, California, on June 1.
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Terrence Floyd, George Floyd's brother, visits a makeshift memorial in Minneapolis on June 1. "He was barely able to walk," CNN's Sara Sidner reported. "He had to have two people on either side of him holding him up as he tried to make his way to the spot." He later spoke to the crowd and called for peace.
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Protesters gather in New York's Times Square on June 1.
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Protesters burn materials during a protest in Washington, DC, early on June 1.
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Police stand guard outside the White House as people gather to protest on May 31.
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A woman is carried by police in Charlotte, North Carolina, on May 31.
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Jeffrey Maddrey, an assistant chief of the New York Police Department, takes a knee during a rally in Brooklyn on May 31.
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A protester kneels in front of a police line in Memphis, Tennessee, on May 31.
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A person is seen inside a damaged 7-Eleven store in New York on May 31.
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Protester Kendrick Cutkelvin uses a SWAT vehicle loudspeaker to disperse a crowd of protesters after a rally in Savannah, Georgia, on May 31.
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A tractor-trailer drives into a crowd of protesters in Minneapolis on May 31. As the driver tried to speed up, protesters overtook the vehicle and the driver stopped, video shows.
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Motorists are ordered to the ground by police during a protest in Minneapolis on May 31.
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People stage a "die-in" protest in Portland, Oregon, on May 31.
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Police react to demonstrators near the White House on May 31.
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A demonstrator jumps on a police car in Washington, DC.
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A woman is helped after being hit with pepper spray in Minneapolis.
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Police confront protesters at the Barclays Center in New York on May 31.
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A young boy raises his fist during a demonstration in Atlanta on May 31.
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Demonstrators pray during a march in Atlanta.
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A police officer aims a nonlethal weapon as protesters raise their hands in Santa Monica, California, on May 31.
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A man screams as he sees a police officer take a knee near the White House on May 31.
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Protesters march during a rally in Fort Lauderdale, Florida, on May 31.
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Demonstrators gather to protest near the White House on May 31.
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People demonstrate outside the Minnesota Capitol, which was ringed with state troopers and the National Guard on May 31.
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Armored vehicles from the Minnesota Army National Guard surround the Capitol in St. Paul on May 31.
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Shaynna Ford stands in front of police in Washington, DC, on May 30.
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A protester runs past burning cars and buildings in St. Paul on May 30.
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Protesters link arms in Charlotte on May 30.
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A firework explodes by a police line near the White House on May 30.
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Looters ransack an Urban Outfitters store in Seattle on May 30.
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Police push people back as they detain a protester in Las Vegas on May 30.
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A Los Angeles Police Department kiosk burns in The Grove shopping center during a protest on May 30.
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Protesters hold up metal gates as they build a barrier on a Las Vegas roadway on May 30.
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Bridges over the Chicago River are lifted to limit transportation to and from the Loop, where protesters clashed with police on May 30.
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Visual journalist Ed Ou is seen bleeding after police fired tear gas and rubber bullets in Minneapolis on May 30.
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Police advance through smoke and tear gas in Minneapolis on May 30.
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A protester confronts a police officer in Tampa, Florida, on May 30.
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Protesters begin to kneel during a protest in New York's Queens borough on May 30.
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A demonstrator is injured during a protest near the White House on May 30.
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Police officers kneel during a rally in Coral Gables, Florida on May 30,.
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Protesters jump on an overturned car near the Municipal Services Building in Philadelphia on May 30.
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A police officer shoots rubber bullets at protesters who were throwing rocks and water bottles during a demonstration in Miami on May 30.
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Police stand guard near the CNN Center and Centennial Olympic Park as protests continued in Atlanta on May 30.
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A protester receives first aid after being hit with pepper spray outside the Statehouse in Columbus, Ohio, on May 30.
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Demonstrators clash near the White House on May 30.
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People with signs and masks that read "I can't breathe" attend a protest in Chicago on May 30.
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A protester holds a sign while a vehicle burns in a Philadelphia street on May 30.
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People gather on top of a baseball backstop during a protest in Los Angeles on May 30.
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Thousands of people stage a "die-in" protest at the Colorado State Capitol in Denver on May 30.
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Genesee County Sheriff Chris Swanson high-fives a woman who called his name as he marches with protesters in Flint, Michigan, on May 30.
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Protesters march near the Salt Lake City Police Department on May 30.
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Protesters chant outside Dallas City Hall on May 30.
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People vandalize a Walgreens store during protests in Oakland, California, on May 29.
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Police officers move forward to clear a street during a protest in downtown Los Angeles on May 29.
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Police officers fire tear gas at protesters in Denver on May 29.
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Police officers and protesters clash near the CNN Center in Atlanta on May 29.
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A man walks away as a car burns in a Minneapolis parking garage on May 29.
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Protesters block a street in Los Angeles on May 29.
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Police form a line near the Centennial Olympic Park and CNN Center in Atlanta on May 29.
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Men raise their fists after making their way onto Interstate 75 and stopping traffic in Cincinnati on May 29.
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People in Minneapolis attempt to extinguish burning cars on May 29.
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A protester is detained by police in Houston on May 29.
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Protesters confront police officers while blocking the 110 Freeway in Los Angeles on May 29.
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Protesters walk through downtown Lexington, Kentucky, on May 29.
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Demonstrators in Oakland climb atop a truck while blocking all lanes of traffic on Interstate 880 on May 29.
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Protesters burn a flag outside the CNN Center in Atlanta on May 29.
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A protester in Minneapolis douses himself with milk on May 29.
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A man stands on top of a burning police car during a protest outside the CNN Center in Atlanta on May 29.
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Demonstrators walk along Pennsylvania Avenue in Washington, DC, on May 29.
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A police officer in Boston holds down a protester while another officer uses pepper spray on May 29.
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Protesters chant in Civic Center Park during a rally in Denver on May 29.
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Protesters gather in Kansas City, Missouri, on May 29.
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Demonstrators protest outside CNN headquarters in Atlanta on May 29.
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Protesters ride in cars during a demonstration in Louisville, Kentucky, on May 29.
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Protesters gather in front of a New York City courthouse and jail on May 29.
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A protester holds up an American flag in New Orleans on May 29.
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Jamela J. Pettiford sings during a protest outside the Hennepin County Government Center in Minneapolis on May 29.
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CNN journalist Omar Jimenez is taken into custody during a live broadcast May 29 at the site of protests in Minneapolis. Jimenez's crew, including a producer and a camera operator, were also placed in handcuffs. They have since been released from custody. Police told the crew they were being detained because they were told to move and didn't, one member of the CNN crew relayed to the network. Jimenez could be seen holding his CNN badge while reporting, identifying himself as a reporter and telling the officers the crew would move wherever officers needed them to. Gov. Tim Walz apologized for the arrests and said he took full responsibility.
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Police gather along Minneapolis' Lake Street early on May 29 as fires burned after a night of unrest.
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Protesters gather in front of a burning fast-food restaurant in Minneapolis on May 29.
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Protesters link arms and surround a police officer to protect him from the crowd in Louisville on May 28.
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A protester carries an American flag upside down next to a burning building in Minneapolis on May 28.
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A protester in Memphis winces in pain after being hit with pepper spray by police on May 28.
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A protester dumps fuel on a fire at a Minneapolis police precinct on May 28.
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People demonstrate outside of a burning Minneapolis police precinct on May 28.
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A protester moves around the smoke-filled police precinct in Minneapolis on May 28.
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A woman yells at a sheriff's deputy during a protest in Minneapolis on May 28.
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Crews work to put out fires after an apartment building under construction was burned to the ground during protests in Minneapolis on May 28.
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A protester is detained by police during a rally in New York City's Union Square on May 28.
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Tony L. Clark holds up a poster of George Floyd during a protest in Minneapolis on May 28.
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Protesters speak to police officers during a demonstration in New York City on May 28.
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A protester reacts amid a cloud of tear gas in St. Paul on May 28.
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Hundreds of people listen to speakers May 28 outside of Cup Foods in Minneapolis. The neighborhood grocery store is where police first encountered Floyd.
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People protest in Minneapolis on May 28.
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Gwen Dumas is consoled near a convenience store in Minneapolis on May 28.
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People look on as a construction site burns in Minneapolis on May 27.
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Demonstrators gather in Memphis on May 27.
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A firework explodes as a fire burns inside an Auto Zone store in Minneapolis on May 27.
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People use garden hoses and buckets to save homes in Minneapolis after rioters set fire to a housing complex under construction on May 27.
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Protesters raise their hands up as they react to tear gas during a demonstration in Minneapolis on May 27.
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People join hands across a freeway in Los Angeles during a protest on May 27.
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Protesters use shopping carts as a barricade as they confront police near a Minneapolis police precinct on May 27.
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Demonstrators in Minneapolis raise their hands during a standoff with police on May 27.
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Two police officers stand on the roof of a Minneapolis police precinct during demonstrations on May 27.
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A protester holds up his hands in Minneapolis on May 27.
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Minneapolis protesters react as a projectile launched by police explodes near them on May 27.
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Minneapolis police stand by an illustration of Floyd as demonstrators rally on May 27.
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Protesters gather in Los Angeles on May 27.
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Kika Villareal, left, and her daughter Aubrie march with fellow protesters in Los Angeles on May 27.
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A man facing a row of police officers holds a burnt US flag as protesters gather in downtown Los Angeles on May 27.
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Protesters gather near the Minneapolis Police Third Precinct on May 26.
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Demonstrators march on Minneapolis' Hiawatha Avenue on May 26.
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Demonstrators react in Minneapolis on May 26.
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A police officer throws a tear-gas canister toward protesters during a rally in Minneapolis on May 27.
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Three women join hands in Minneapolis as they pray around a makeshift memorial for Floyd on May 26.
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Protesters and police face off during a rally in Minneapolis on May 26.
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Crowds gather in the street at a protest in Minneapolis on May 26.
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People gather outside a police precinct during demonstrations in Minneapolis on May 26.
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Tear gas is fired as protesters clash with police in Minneapolis on May 26.
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A protester is doused with milk after exposure to tear gas in Minneapolis on May 26.
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People gather and pray around a makeshift memorial in Minneapolis on May 26. It was near the site where Floyd was taken into police custody the previous day.
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People stand outside the Minneapolis Police Third Precinct and chant "Hands up, don't shoot" on May 26.
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A Minneapolis demonstrator holds a sign reading "Justice for George" on May 26.
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Police try to disperse crowds in Minneapolis on May 26.
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Protesters rally around a damaged police vehicle in Minneapolis on May 26.
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Milk is poured on the face of a protester who had been exposed to tear gas in Minneapolis on May 26.
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A car in Minneapolis is hit with tear gas on May 26.
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Crowds rally in Minneapolis on May 26.
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Minneapolis protesters gather in the rain on May 26.
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Floyd's girlfriend, Shawanda Hill, is in the center of this photo taken on May 26, near the spot where he died.
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The maneuver is 'not a neck restraint'
The Minneapolis Police Department allows arresting officers to use two types of neck restraints on a subject, but only officers trained in them can use them, according to the department's Policy & Procedure Manual.
Conscious neck restraint involves putting light pressure on the subject's neck with an arm or leg without cutting off their airway, and unconscious neck restraint puts enough pressure on a subject to make them lose consciousness without killing them, according to the manual.
Both can only be conducted when a subject is resisting arrest, per the policy, and unconscious neck restraint is allowed only when subjects are aggressively resisting and can't be subdued in another manner.
The method used to restrain Floyd doesn't fit neatly into either of those categories, Stoughton said.
"This is not a neck restraint," he said of the position Floyd was held in during his arrest. "It's not just putting pressure on someone's neck. It's really dangerous."
The Minneapolis Police Department didn't return CNN's requests for comment.
Police aren't usually taught the move in training
The choice of restraint didn't fall in line with what police are taught in training, said Montgomery County Police Captain Sonia Pruitt.
"You use the force necessary to have someone comply and take them into custody," Pruitt, who is chairwoman of the National Black Police Association, said. "This man was already in handcuffs and already on the ground. (Restraining him) was total excessive force."
Pruitt said a few years ago she started asking members of other police departments whether they'd been taught the knee-to-neck method of restraint because she'd seen it being repeatedly used, particularly with black suspects.
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She said almost everyone she talked with denied that they were trained in the maneuver because of the high risk of injury.
