Flashback: Boston Marathon bombings

Runners run towards the finish line of the Boston Marathon as an explosion erupts near the finish line of the race in Boston, Massachusetts, April 15, 2013. REUTERS/Dan Lampariello

Blood and debris are seen on the sidewalk along Boylston Street a day after the explosions April 16, 2013. REUTERS/Jessica Rinaldi

A woman is comforted by a man near a triage tent April 15, 2013. REUTERS/Jessica Rinaldi

Boston Marathon bomb scene pictures taken by investigators show the remains of an explosive device. REUTERS

A still image captured from surveillance video shows the moment when a second bomb exploded near the finish line, April 15, 2013. A circle (top) indicates the area of a man identified by prosecutors as Dzhokhar Tsarnaev. REUTERS/U.S. Attorney's...more

Boston Marathon bomb scene pictures taken by investigators show the remains of an explosive device. REUTERS

Officials take crime scene photos a day after the explosions April 16, 2013. REUTERS/Shannon Stapleton

The accused, Dzokhar (L) and Tamerlan Tsarnaev, are seen in handout photo released through the FBI website, April 18, 2013. REUTERS/FBI/Handout

Dzokhar (L) and Tamerlan Tsarnaev are shown in this handout photo during an FBI news conference in Boston, April 18, 2013. REUTERS/FBI/Handout

A runner is escorted from the scene after explosions went off at the Boston Marathon, April 15, 2013. REUTERS/Jessica Rinaldi

Blood in seen on the sidewalk in front of a candy store advertising a Marathon Monday sale a day after the explosions in Boston, April 16, 2013. REUTERS/Jessica Rinaldi

Bloody clothing and medical pads are seen on Commonwealth Avenue near the scene of the explosions in Boston, April 15, 2013. REUTERS/Scott Eisen

An investigator carries an evidence bag off a rooftop near the site of two explosions at the finish line of the Boston Marathon, April 17, 2013. REUTERS/Brian Snyder

Unclaimed Boston Marathon runners' belongings are seen, April 16, 2013. REUTERS/Shannon Stapleton

A U.S. national guardsman waits in a school bus near the finish line of the Boston Marathon, April 15, 2013. REUTERS/Shannon Stapleton

A member of the SWAT team motions to a resident to come out of the house as they conduct a house to house search for Dzhokar Tsarnaev in Watertown, Massachusetts April 19, 2013. REUTERS/Jessica Rinaldi

A family reacts after police SWAT teams assaulted a house on their street, firing their weapons on Franklin Street during the search for Dzhokhar Tsarnaev in Watertown, Massachusetts April 19, 2013. REUTERS/Jim Bourg

Law enforcement officers place themselves in an overhead position on Arsenal Street, in the search area for Dzhokar Tsarnaev in Watertown, Massachusetts April 19, 2013. REUTERS/Lucas Jackson

A law enforcement bomb technician walks away after preparing the controlled detonation of a suspicious object in Watertown, Massachusetts April 19, 2013. REUTERS/Lucas Jackson

Members of the media take cover on the instruction of law enforcement officers while covering a police reaction to a suspect on Arsenal Street, in the search area for Dzhokar Tsarnaev, in Watertown, Massachusetts April 19, 2013. REUTERS/Lucas Jackson

Police officers move towards a police assault on a house with their guns drawn as gunfire erupts on Franklin Street during the search for Dzhokhar Tsarnaev in Watertown, Massachusetts April 19, 2013. REUTERS/Jim Bourg

An FBI officer stands in front of the boat at 67 Franklin St. where Dzhokhar Tsarnaev was hiding inside in Watertown, Massachusetts, April 19, 2013. REUTERS/Lucas Jackson

An aerial infrared image shows the outline of Dzhokhar Tsarnaev in a boat during the manhunt in Watertown, Massachusetts, April 19, 2013. REUTERS/Massachusetts State Police/Handout

A blood-stained message that prosecutors say Dzhokhar Tsarnaev wrote on the inside of a boat is seen with bullet holes in an evidence picture. REUTERS/U.S. Department of Justice/Handout

Dzhokhar Tsarnaev is seen standing in a boat where he was apprehended by police, in an image entered as evidence. REUTERS/U.S. Attorney's Office/Handout

Dzhokhar Tsarnaev, 19, is searched by law enforcement officers in Watertown, Massachusetts, April 19, 2013. REUTERS/Massachusetts State Police/Handout

Dzhokhar Tsarnaev is pictured in this handout photo presented as court evidence. REUTERS/U.S. Attorney's Office in Boston/Handout

Members of the public cheer as police officers leave the scene where Dzhokhar Tsarnaev was taken into custody in Watertown, Massachusetts April 19, 2013. REUTERS/Brian Snyder

A tactical police officer gives the thumbs-up as he emerges from the site where police captured Dzhokhar Tsarnaev in Watertown, Massachusetts April 19, 2013. REUTERS/Brian Snyder

People wave U.S. flags while cheering as police drive down Arlington Street in Watertown, Massachusetts April 19, 2013. REUTERS/Shannon Stapleton

Colored flags mark evidence in the yard where Dzhokhar Tsarnaev was hiding at 67 Franklin St. in Watertown, Massachusetts, April 20, 2013. REUTERS/Lucas Jackson

A photo of bombing victim Lu Lingzi, a Boston University graduate student and Chinese citizen, is seen outside the Boston University Marsh Chapel before her memorial in Boston, April 17, 2013. REUTERS/Shannon Stapleton

People attend the interfaith memorial service at the Cathedral of the Holy Cross for the victims of the Boston Marathon bombing in Boston, April 18, 2013. REUTERS/Kevin Lamarque

A tribute to MIT police officer Sean Collier (L), killed by the Boston marathon bombing suspects and MBTA police officer Richard Donahue, Jr. (R), shot and injured in a shootout with the suspects, is seen at a memorial to the victims of the Boston...more

Boston Bruins Dennis Seidenberg observes a moment of silence for the victims of the bombings before the start of a hockey game against the Buffalo Sabres in Boston April 17, 2013. REUTERS/Jessica Rinaldi

Candles are lit for those who died in the Boston Marathon bombings and the subsequent police manhunt at a memorial on Boylston Street in Boston, Massachusetts April 21, 2013. REUTERS/Brian Snyder

A participant number tag is seen among running shoes left at the makeshift memorial, in an exhibit titled "Dear Boston: Messages from the Marathon Memorial" at the Boston Public Library, April 16, 2014. REUTERS/Brian Snyder

Boston Cardinal Sean O'Malley (L) joins the family of Boston Marathon bombing victim Martin Richard at the finish line for a wreath-laying ceremony in Boston, Massachusetts April 15, 2014. Martin Richard's sister Jane (5th R) wipes her face as she...more

Crosses for Boston Marathon bombing victims Krystle Campbell, Lu Lingzi and Martin Richard, artifacts saved from the makeshift Boston Marathon bombing memorial, are seen at the Iron Mountain storage facility in Northborough, Massachusetts March 28,...more

A Boston Marathon bombing survivor receives a hug next to the site of the first bomb explosion on Boylston Street in Boston, Massachusetts April 24, 2013. REUTERS/Brian Snyder

Boston bombing survivor Jeff Bauman stands with his rescuer Carlos Arredondo (L) as they were honored before the start of a playoff hockey game between the Bruins and the Chicago Blackhawks in Boston, June 24, 2013. REUTERS/Pool/Dave Sandford

Runners holding American flags and a Chinese flag cross the finish line after completing the final mile of the Boston Marathon course during "#onerun" in Boston, Massachusetts, May 25, 2013. The event was organized to give athletes and spectators an...more

Runner's shoes, artifacts saved from the makeshift Boston Marathon bombing memorial, are seen at the Iron Mountain storage facility in Northborough, Massachusetts March 28, 2014. REUTERS/Brian Snyder

Survivors Celeste Corcoran (C) and her daughter Sydney (R) finish the race with Celeste's sister Carmen Acabbo, who ran the 118th Boston Marathon, in Boston, Massachusetts April 21, 2014. REUTERS/Brian Snyder

The shoes of bombing survivor J.P. Norden read "Boston Strong" as he stands at the finish line on the one-year anniversary of the bombings in Boston, Massachusetts April 15, 2014. REUTERS/Brian Snyder