Runners in the men's elite race of the 2013 Virgin London Marathon observe 30 seconds of silence in honor of the victims of the Boston Marathon bombings before the start of the race in London on Sunday, April 21. People worldwide expressed their condolences after at least three people were killed and scores wounded in two explosions Monday near the finish line of the Boston Marathon. <a href="index.php?page=&url=http%3A%2F%2Fwww.cnn.com%2FSPECIALS%2Fus%2Fboston-bombings-galleries%2Findex.html">See all photography related to the Boston bombings.</a>
Boston bombings: The world reacts —
Runners in the men's elite race of the 2013 Virgin London Marathon observe 30 seconds of silence in honor of the victims of the Boston Marathon bombings before the start of the race in London on Sunday, April 21. People worldwide expressed their condolences after at least three people were killed and scores wounded in two explosions Monday near the finish line of the Boston Marathon. See all photography related to the Boston bombings.
CARL COURT/AFP/Getty Images
A competitor at the London Marathon inscribed the word "Boston" into his hair at the London Marathon on April 21.
Boston bombings: The world reacts —
A competitor at the London Marathon inscribed the word "Boston" into his hair at the London Marathon on April 21.
Chris Jackson/Getty Images
A man carries a U.S. flag as he runs with other marathoners during the London Marathon on April 21.
Boston bombings: The world reacts —
A man carries a U.S. flag as he runs with other marathoners during the London Marathon on April 21.
ANDY RAIN/EPA/LANDOV
White balloons are released in remembrance of the Boston Marathon bombings victims before the commencement of the 5K San Isidro Run in Lima, Peru, on April 21.
Boston bombings: The world reacts —
White balloons are released in remembrance of the Boston Marathon bombings victims before the commencement of the 5K San Isidro Run in Lima, Peru, on April 21.
MARIANA BAZO/REUTERS/LANDOV
FC Barcelona's players link up during a minute of silence in memory of those killed in the Boston Marathon bombings before the Spanish league football match against Levante UD in Barcelona on Saturday, April 20.
Boston bombings: The world reacts —
FC Barcelona's players link up during a minute of silence in memory of those killed in the Boston Marathon bombings before the Spanish league football match against Levante UD in Barcelona on Saturday, April 20.
JOSEP LAGO/AFP/Getty Images
Runners from the Boston 4:09 group celebrate as they make their way to the finish line at the Salt Lake City Marathon on Friday.
Boston bombings: The world reacts —
Runners from the Boston 4:09 group celebrate as they make their way to the finish line at the Salt Lake City Marathon on Friday.
George Frey/Getty Images
Bryant Jensen, wearing a shirt honoring the Boston bombing victims, wins the Salt Lake City Marathon on Friday.
Boston bombings: The world reacts —
Bryant Jensen, wearing a shirt honoring the Boston bombing victims, wins the Salt Lake City Marathon on Friday.
George Frey/Getty Images
Vice President Joe Biden speaks to reporters about the bombings at the Boston Marathon in Washington on Tuesday, April 16.
Boston bombings: The world reacts —
Vice President Joe Biden speaks to reporters about the bombings at the Boston Marathon in Washington on Tuesday, April 16.
SAUL LOEB/AFP/Getty Images
French soldiers patrol the Eiffel Tower in Paris on April 16. Security in public areas and events was stepped up after the bombings.
Boston bombings: The world reacts —
French soldiers patrol the Eiffel Tower in Paris on April 16. Security in public areas and events was stepped up after the bombings.
FRED DUFOUR/AFP/Getty Images
Arsenal and Everton soccer teams observe a moment of silence for the Hillsborough disaster anniversary and the victims of the Boston terror blasts during the Barclays Premier League on April 16, in London, England. In 1989 nearly 100 people were killed and several hundred injured due to overcrowding at a match at the Hillsborough Stadium in Sheffield, England.
Boston bombings: The world reacts —
Arsenal and Everton soccer teams observe a moment of silence for the Hillsborough disaster anniversary and the victims of the Boston terror blasts during the Barclays Premier League on April 16, in London, England. In 1989 nearly 100 people were killed and several hundred injured due to overcrowding at a match at the Hillsborough Stadium in Sheffield, England.
Michael Regan/Getty Images
A man puts flowers near the U.S. Embassy in Moscow on Tuesday.
Boston bombings: The world reacts —
A man puts flowers near the U.S. Embassy in Moscow on Tuesday.
YURI KOCHETKOV/epa/Landov
During a committee session in Tokyo on Tuesday, Japanese Prime Minister Shinzo Abe said that police have stepped up security at key domestic facilities after the Boston explosions.
Boston bombings: The world reacts —
During a committee session in Tokyo on Tuesday, Japanese Prime Minister Shinzo Abe said that police have stepped up security at key domestic facilities after the Boston explosions.
Kosovo's foreign minister, Enver Hoxhaj, expressed sympathy for the victims of the Boston explosions following talks with German Foreign Minister Guido Westerwelle in Berlin on Tuesday.
Boston bombings: The world reacts —
Kosovo's foreign minister, Enver Hoxhaj, expressed sympathy for the victims of the Boston explosions following talks with German Foreign Minister Guido Westerwelle in Berlin on Tuesday.
JOHN MACDOUGALL/AFP/Getty Images
Members of the U.S. Army National Guard Joint Task Force Empire Shield patrol Grand Central Terminal in New York on Tuesday.
Boston bombings: The world reacts —
Members of the U.S. Army National Guard Joint Task Force Empire Shield patrol Grand Central Terminal in New York on Tuesday.
CARLO ALLEGRI/reuters/landov
A message asking passengers to report suspicious activity flashes on an electronic sign inside a New York City subway station on Tuesday.
Boston bombings: The world reacts —
A message asking passengers to report suspicious activity flashes on an electronic sign inside a New York City subway station on Tuesday.
BRENDAN MCDERMID/REUTERS/LANDOV
Speaker of the House John Boehner makes remarks after a House GOP caucus meeting on Capitol Hill on Tuesday.
Boston bombings: The world reacts —
Speaker of the House John Boehner makes remarks after a House GOP caucus meeting on Capitol Hill on Tuesday.
Chip Somodevilla/Getty Images
Commuter passengers move through New York's Penn Station on April 16.
Boston bombings: The world reacts —
Commuter passengers move through New York's Penn Station on April 16.
John Moore/Getty Images
Massachusetts Gov. Deval Patrick speaks at a press conference about the bombings on Tuesday. From left, Sen. Elizabeth Warren, FBI Special Agent in charge of Boston Richard Deslauriers, and Boston Mayor Thomas Menino, seated at front left.
Boston bombings: The world reacts —
Massachusetts Gov. Deval Patrick speaks at a press conference about the bombings on Tuesday. From left, Sen. Elizabeth Warren, FBI Special Agent in charge of Boston Richard Deslauriers, and Boston Mayor Thomas Menino, seated at front left.
STAN HONDA/AFP/Getty Images
A police K-9 unit dog stands by as train passengers pass through Penn Station on Tuesday in New York.
Boston bombings: The world reacts —
A police K-9 unit dog stands by as train passengers pass through Penn Station on Tuesday in New York.
John Moore/Getty Images
The bombings dominate the front pages of newspapers at a newsstand on Newbury Street in Boston.
Boston bombings: The world reacts —
The bombings dominate the front pages of newspapers at a newsstand on Newbury Street in Boston.
Darren McCollester/Getty Images
Members of the Calgary Flames observe a moment of silence before the start of their National Hockey League game against the Minnesota Wild in Calgary, Alberta, on Monday, April 15.
Boston bombings: The world reacts —
Members of the Calgary Flames observe a moment of silence before the start of their National Hockey League game against the Minnesota Wild in Calgary, Alberta, on Monday, April 15.
TODD KOROL/reuters/LANDOV
President Barack Obama makes a statement Monday evening.
Boston bombings: The world reacts —
President Barack Obama makes a statement Monday evening.
Mark Wilson/Getty Images
Police officers stand in front of the White House on Monday after the Secret Service heightened security.
Boston bombings: The world reacts —
Police officers stand in front of the White House on Monday after the Secret Service heightened security.
Arno Burgi/dpa/landov
A New York police officer inspects a man's bag in Grand Central Terminal in Manhattan on Monday.
Boston bombings: The world reacts —
A New York police officer inspects a man's bag in Grand Central Terminal in Manhattan on Monday.
Mario Tama/Getty Images
A police officer mans his post at New York's Times Square on Monday.
Boston bombings: The world reacts —
A police officer mans his post at New York's Times Square on Monday.
EMMANUEL DUNAND/AFP/Getty Images
Cars stop at a checkpoint at Los Angeles International Airport on Monday.
Boston bombings: The world reacts —
Cars stop at a checkpoint at Los Angeles International Airport on Monday.
Kevork Djansezian/Getty Images
A bomb squad vehicle is parked at the Los Angeles International Airport departures terminal on Monday.
Boston bombings: The world reacts —
A bomb squad vehicle is parked at the Los Angeles International Airport departures terminal on Monday.
Kevork Djansezian/Getty Images
A police officer and his dog guard a Long Island Rail Road train at the station in Hicksville, New York, on Monday. <a href="index.php?page=&url=http%3A%2F%2Fwww.cnn.com%2FSPECIALS%2Fus%2Fboston-bombings-galleries%2Findex.html">See all photography related to the Boston bombings.</a>
Boston bombings: The world reacts —
A police officer and his dog guard a Long Island Rail Road train at the station in Hicksville, New York, on Monday. See all photography related to the Boston bombings.
Bruce Bennett/Getty Images