It has been 20 years since the September 11 attacks.

Across the country, people are pausing to honor the victims and reflect on the day.

Nearly 3,000 people were killed when hijackers crashed planes into the World Trade Center, the Pentagon and a field in Pennsylvania.

"These memorials are really important," said President Joe Biden, who visited all three sites. "But they're also incredibly difficult for the people affected by them, because it brings back the moment they got the phone call. It brings back the instant they got the news, no matter how years go by."

People observe the Tribute in Light memorial in New York from the top of a parking garage.
People observe the Tribute in Light memorial in New York from the top of a parking garage.
Ashley Gilbertson/VII/Redux for CNN
A bus makes its way through New York's Chinatown. The destination sign was changed to read "9/11 We Remember."
A bus makes its way through New York's Chinatown. The destination sign was changed to read "9/11 We Remember."
Ashley Gilbertson/VII/Redux for CNN
People watch a ceremony being held Saturday night in Chinatown in honor of Betty Ann Ong, the American Airlines flight attendant who alerted authorities to the hijacking before the first plane hit the World Trade Center.
People watch a ceremony being held Saturday night in Chinatown in honor of Betty Ann Ong, the American Airlines flight attendant who alerted authorities to the hijacking before the first plane hit the World Trade Center.
Ashley Gilbertson/VII/Redux for CNN
A child visits the Washington Monument as flags are flown at half-staff and a beam of light is cast into the sky from the Pentagon in honor of 9/11.
A child visits the Washington Monument as flags are flown at half-staff and a beam of light is cast into the sky from the Pentagon in honor of 9/11.
Sarah Silbiger for CNN
Vianka Brito, her brother Javier Brito, and Ariel Mercedes visit Ground Zero. They are all former New Yorkers who now live in Rhode Island.
Vianka Brito, her brother Javier Brito, and Ariel Mercedes visit Ground Zero. They are all former New Yorkers who now live in Rhode Island.
Ashley Gilbertson/VII/Redux for CNN
Four sisters make a charcoal imprint of the name of their cousin Richard Avery Aronow at the memorial in New York.
Four sisters make a charcoal imprint of the name of their cousin Richard Avery Aronow at the memorial in New York.
Anthony Behar/Pool/Getty Images
John Roberts leads community members in reading victims' names during a service at the 9/11 Breezy Point Memorial in New York at the time the first plane struck the North Tower of the World Trade Center. The cross is made of steel from the collapsed building.
John Roberts leads community members in reading victims' names during a service at the 9/11 Breezy Point Memorial in New York at the time the first plane struck the North Tower of the World Trade Center. The cross is made of steel from the collapsed building.
Caitlin Ochs/Reuters
Firefighters and their family members attend a memorial service for fallen firefighters at St. Patrick's Cathedral in New York.
Firefighters and their family members attend a memorial service for fallen firefighters at St. Patrick's Cathedral in New York.
Go Nakamura/Reuters
A man lays down to observe the<strong> </strong>2,977<strong> </strong>American flags placed on the National Mall in Washington, DC, representing the lives lost on 9/11, not including the hijackers.
A man lays down to observe the 2,977 American flags placed on the National Mall in Washington, DC, representing the lives lost on 9/11, not including the hijackers.
Sarah Silbiger for CNN
Divers carry a folded American flag underwater Saturday in the Florida Keys National Marine Sanctuary off Key West, Florida. The flag was unfurled on the Gen. Hoyt S. Vandenberg shipwreck in commemoration of 9/11. The Vandenberg, a 523-foot-long former Army troop transporter and Navy missile-tracking ship, was scuttled in 2009 as an artificial reef.
Divers carry a folded American flag underwater Saturday in the Florida Keys National Marine Sanctuary off Key West, Florida. The flag was unfurled on the Gen. Hoyt S. Vandenberg shipwreck in commemoration of 9/11. The Vandenberg, a 523-foot-long former Army troop transporter and Navy missile-tracking ship, was scuttled in 2009 as an artificial reef.
Joe Berg/Florida Keys News Bureau/AP
Crew members of the USS Somerset attend the memorial ceremony at the Flight 93 National Memorial near Shanksville, Pennsylvania.
Crew members of the USS Somerset attend the memorial ceremony at the Flight 93 National Memorial near Shanksville, Pennsylvania.
Maddie McGarvey for CNN
Defense Secretary Lloyd Austin, left, and Chairman of the Joint Chiefs of Staff Gen. Mark Milley greet each other during the ceremony at the Pentagon.
Defense Secretary Lloyd Austin, left, and Chairman of the Joint Chiefs of Staff Gen. Mark Milley greet each other during the ceremony at the Pentagon.
Sarah Silbiger for CNN
To commemorate the anniversary of 9/11, military planes fly over Memorial Stadium in Lincoln, Nebraska, as the national anthem is played before Nebraska faced Buffalo in a college-football game Saturday.
To commemorate the anniversary of 9/11, military planes fly over Memorial Stadium in Lincoln, Nebraska, as the national anthem is played before Nebraska faced Buffalo in a college-football game Saturday.
Rebecca S. Gratz/AP
Victims' family members and loved ones attend the ceremony in New York.
Victims' family members and loved ones attend the ceremony in New York.
Chip Somodevilla/Getty Images
A jogger holds an American flag as he runs past the Washington Monument on Saturday.
A jogger holds an American flag as he runs past the Washington Monument on Saturday.
Anna Moneymaker/Getty Images
President Joe Biden and first lady Jill Biden participate in a wreath-laying ceremony at the Pentagon.
President Joe Biden and first lady Jill Biden participate in a wreath-laying ceremony at the Pentagon.
Alex Brandon/AP
Flowers are left for the victims of Flight 93 at the memorial site near Shanksville.
Flowers are left for the victims of Flight 93 at the memorial site near Shanksville.
Maddie McGarvey for CNN
Former President George W. Bush and former first lady Laura Bush hold hands during the ceremony near Shanksville. "In the weeks and months following the 9/11 attacks, I was proud to lead an amazing, resilient, united people," Bush said.
Former President George W. Bush and former first lady Laura Bush hold hands during the ceremony near Shanksville. "In the weeks and months following the 9/11 attacks, I was proud to lead an amazing, resilient, united people," Bush said.
Mandel Ngan/AFP/Getty Images
Mennonites from western Massachusetts sing hymns at Ground Zero.
Mennonites from western Massachusetts sing hymns at Ground Zero.
Ashley Gilbertson/VII/Redux for CNN
New York firefighters line the street during a ceremony marking when the first plane hit the World Trade Center.
New York firefighters line the street during a ceremony marking when the first plane hit the World Trade Center.
Ashley Gilbertson/VII/Redux for CNN
Members of the US Marine Band observe a moment of silence before performing at the Flight 93 National Memorial near Shanksville. Forty passengers and crew members aboard United Airlines Flight 93 died when the plane crashed into a field.
Members of the US Marine Band observe a moment of silence before performing at the Flight 93 National Memorial near Shanksville. Forty passengers and crew members aboard United Airlines Flight 93 died when the plane crashed into a field.
Gene J. Puskar/AP
Bells are rung during the ceremony near Shanksville in honor of the passengers and crew members who died on 9/11.
Bells are rung during the ceremony near Shanksville in honor of the passengers and crew members who died on 9/11.
Maddie McGarvey for CNN
Katie Mascali is comforted by her fiance, Andre Jabban, as they stand near the name of her father, Joseph Mascali, during the ceremony in New York.
Katie Mascali is comforted by her fiance, Andre Jabban, as they stand near the name of her father, Joseph Mascali, during the ceremony in New York.
Craig Ruttle/Pool/Getty Images
A flower is seen at the edge of one of the reflecting pools at the 9/11 memorial in New York.
A flower is seen at the edge of one of the reflecting pools at the 9/11 memorial in New York.
Ashley Gilbertson/VII/Redux for CNN
Firefighters pause for a moment of silence as they participate in the Colorado 9/11 Memorial Stair Climb at the Red Rocks Park and Amphitheatre in Morrison, Colorado. Participants walk nine laps around the Red Rocks Amphitheater, which is equivalent to the 110 floors of the World Trade Center.
Firefighters pause for a moment of silence as they participate in the Colorado 9/11 Memorial Stair Climb at the Red Rocks Park and Amphitheatre in Morrison, Colorado. Participants walk nine laps around the Red Rocks Amphitheater, which is equivalent to the 110 floors of the World Trade Center.
Chet Strange/AFP/Getty Images
Marian Brennan sits at the headstone of her nephew Lance Cpl. Julian T. Brennan in Section 60 of Arlington National Cemetery, reserved for veterans of recent US conflicts.
Marian Brennan sits at the headstone of her nephew Lance Cpl. Julian T. Brennan in Section 60 of Arlington National Cemetery, reserved for veterans of recent US conflicts.
Michael A. McCoy/Reuters
Former President Donald Trump visits the 17th Precinct of the New York City Police Department.
Former President Donald Trump visits the 17th Precinct of the New York City Police Department.
Mario Anzuoni/Reuters
A member of the US Air Force walks through the National 9/11 Pentagon Memorial on Saturday.
A member of the US Air Force walks through the National 9/11 Pentagon Memorial on Saturday.
Sarah Silbiger for CNN
Mourners embrace during the ceremony in New York.
Mourners embrace during the ceremony in New York.
Jonathan Ernst/Reuters
Firefighters salute on top of a firetruck during the ceremony at the Pentagon.
Firefighters salute on top of a firetruck during the ceremony at the Pentagon.
Sarah Silbiger for CNN
Vice President Kamala Harris touches the memorial wall near Shanksville. "What happened on Flight 93 told us then — and it still tells us — so much about the courage of those on board who gave everything they possibly could, about the resolve of the first responders who risked everything, and about the resilience of the American people," Harris said during the ceremony.
Vice President Kamala Harris touches the memorial wall near Shanksville. "What happened on Flight 93 told us then — and it still tells us — so much about the courage of those on board who gave everything they possibly could, about the resolve of the first responders who risked everything, and about the resilience of the American people," Harris said during the ceremony.
Maddie McGarvey for CNN
In Patriot Park near Shanksville, 7,049 flags are planted to honor the service members killed during the wars in Iraq and Afghanistan.
In Patriot Park near Shanksville, 7,049 flags are planted to honor the service members killed during the wars in Iraq and Afghanistan.
Maddie McGarvey for CNN
A young boy holds a flag during the ceremony near Shanksville.
A young boy holds a flag during the ceremony near Shanksville.
Maddie McGarvey for CNN
Loved ones of the passengers and crew members of Flight 93 attend the ceremony near Shanksville.
Loved ones of the passengers and crew members of Flight 93 attend the ceremony near Shanksville.
Maddie McGarvey for CNN
A military chorus participates in the ceremony at the Pentagon.
A military chorus participates in the ceremony at the Pentagon.
Sarah Silbiger for CNN
A man pauses at the Ground Zero memorial.
A man pauses at the Ground Zero memorial.
Ashley Gilbertson/VII/Redux for CNN
A firefighter attends a ceremony Saturday at a fire station in New York. Of those who perished during the initial attacks and the subsequent collapses of the Twin Towers, 343 were New York City firefighters.
A firefighter attends a ceremony Saturday at a fire station in New York. Of those who perished during the initial attacks and the subsequent collapses of the Twin Towers, 343 were New York City firefighters.
Shannon Stapleton/Reuters
New Jersey native Bruce Springsteen performs his song "I'll See You In My Dreams" during the 9/11 ceremony in New York. Springsteen's 2002 album "The Rising" was a response to the attacks.
New Jersey native Bruce Springsteen performs his song "I'll See You In My Dreams" during the 9/11 ceremony in New York. Springsteen's 2002 album "The Rising" was a response to the attacks.
Chip Somodevilla/Getty Images
A sign for the 9/11 Never Forget Project is displayed on the National Mall in Washington, DC. Young America's Foundation, a conservative youth organization, created the project.
A sign for the 9/11 Never Forget Project is displayed on the National Mall in Washington, DC. Young America's Foundation, a conservative youth organization, created the project.
Sarah Silbiger for CNN
Family members and loved ones listen as victims' names are read during the commemoration ceremony in New York. At the World Trade Center, 2,753 people were killed after American Airlines Flight 11 and United Airlines Flight 175 crashed into the North and South Towers.
Family members and loved ones listen as victims' names are read during the commemoration ceremony in New York. At the World Trade Center, 2,753 people were killed after American Airlines Flight 11 and United Airlines Flight 175 crashed into the North and South Towers.
Chip Somodevilla/Getty Images
Flowers and flags are left for the victims of Flight 93 at the memorial near Shanksville.
Flowers and flags are left for the victims of Flight 93 at the memorial near Shanksville.
Maddie McGarvey for CNN
Former President George W. Bush speaks at the memorial ceremony in Shanksville. "On America's day of trial and grief, I saw millions of people instinctively grab for a neighbor's hand and rally to the cause of one another. That is the America I know," Bush said.
Former President George W. Bush speaks at the memorial ceremony in Shanksville. "On America's day of trial and grief, I saw millions of people instinctively grab for a neighbor's hand and rally to the cause of one another. That is the America I know," Bush said.
Maddie McGarvey for CNN
A musician practices ahead of a ceremony Saturday outside the Pentagon in Arlington, Virginia. The American flag is draped over the site of impact where the hijacked plane crashed into the Pentagon, killing 184 people.
A musician practices ahead of a ceremony Saturday outside the Pentagon in Arlington, Virginia. The American flag is draped over the site of impact where the hijacked plane crashed into the Pentagon, killing 184 people.
Sarah Silbiger for CNN
A young girl holds flowers during the ceremony near Shanksville.
A young girl holds flowers during the ceremony near Shanksville.
Maddie McGarvey for CNN
Members of the military salute the Color Guard during the ceremony at the Pentagon.
Members of the military salute the Color Guard during the ceremony at the Pentagon.
Sarah Silbiger for CNN
Flowers are placed at the memorial near Shanksville.
Flowers are placed at the memorial near Shanksville.
Maddie McGarvey for CNN
Family members and first responders from the Tunnel to Towers Foundation make their way to Ground Zero.
Family members and first responders from the Tunnel to Towers Foundation make their way to Ground Zero.
Ashley Gilbertson/VII/Redux for CNN
People gather in Manhattan near Ground Zero on Saturday.
People gather in Manhattan near Ground Zero on Saturday.
Spencer Platt/Getty Images
Retired Fire Chief Dave Dean of Heartland, Wisconsin, visits Ground Zero to mark the anniversary. He helped with search-and-rescue efforts in the aftermath of 9/11.
Retired Fire Chief Dave Dean of Heartland, Wisconsin, visits Ground Zero to mark the anniversary. He helped with search-and-rescue efforts in the aftermath of 9/11.
Ashley Gilbertson/VII/Redux for CNN
Flowers are placed at the name of Frank Spinelli at the New York memorial on Saturday.
Flowers are placed at the name of Frank Spinelli at the New York memorial on Saturday.
Anthony Behar/Pool/AP
A baby holds a lanyard that reads "Family" during the ceremony at the Pentagon, indicating they are family members of a person who died on 9/11.
A baby holds a lanyard that reads "Family" during the ceremony at the Pentagon, indicating they are family members of a person who died on 9/11.
Sarah Silbiger for CNN
Family members pay their respects prior to the ceremony at the Flight 93 National Memorial in Shanksville.
Family members pay their respects prior to the ceremony at the Flight 93 National Memorial in Shanksville.
Jim Lo Scalzo/EPA-EFE/Shutterstock
Flag bearers rehearse before the ceremony in New York on Saturday.
Flag bearers rehearse before the ceremony in New York on Saturday.
John Minchillo/AP
Guests arrive for the ceremony at the Pentagon.
Guests arrive for the ceremony at the Pentagon.
Sarah Silbiger for CNN
A person walks through the Empty Sky 9/11 memorial at Liberty State Park in Jersey City, New Jersey, as the sun rises Saturday behind lower Manhattan.
A person walks through the Empty Sky 9/11 memorial at Liberty State Park in Jersey City, New Jersey, as the sun rises Saturday behind lower Manhattan.
Gary Hershorn/Getty Images
A memorial service is held Friday, September 10, outside the New York Fire Department's 10 House near Ground Zero.
A memorial service is held Friday, September 10, outside the New York Fire Department's 10 House near Ground Zero.
Ashley Gilbertson/VII/Redux for CNN
The memorial at Ground Zero is seen on Friday.
The memorial at Ground Zero is seen on Friday.
Ashley Gilbertson/VII/Redux for CNN