Bombings killed three people and wounded more than 170 at the finish line of the 2013 Boston Marathon. (Warning: Graphic images)
Investigators sift through evidence on Boylston Street just
Investigators sift through evidence on Boylston Street just up from the finish line of the Boston Marathon in Boston. (April 18, 2013) (Credit: AP)
Investigators comb through the post finish line area
Investigators comb through the post finish line area of the Boston Marathon at Boylston Street, two days after two bombs exploded just before the finish line in Boston. (April 17, 2013) (Credit: AP)
An FBI investigator walks down a fire truck
An FBI investigator walks down a fire truck ladder with a bag from the top of a building at the corner of Boylston Street and Fairfield Street. Investigators in white jumpsuits fanned out across the streets, rooftops and awnings around the blast site in search of clues on Wednesday. (April 17, 2013) (Credit: AP )

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This FBI image shows the remains of a
This FBI image shows the remains of a black backpack that the FBI says contained one of the bombs that exploded during the Boston Marathon. (April 15, 2013) (Credit: AP)
The remains of a pressure cooker that investigators
The remains of a pressure cooker that investigators say was part of one of the bombs that exploded during the Boston Marathon is shown in this image provided by the FBI. (April 16, 2013) (Credit: AP)
The moment the second explosion occurred near the
The moment the second explosion occurred near the finish line at the 117th Boston Marathon. The bomb blasts took the lives of three victims and injured more than 260 others. (April 15, 2013) (Credit: Dan Lampariello/DobsonAgency/Rex )
Medical personnel help an injured man near the
Medical personnel help an injured man near the finish line at the 2013 Boston Marathon after an explosion. (April 15, 2013) (Credit: AP)
Bill Iffrig, 78, lies on the ground as
Bill Iffrig, 78, lies on the ground as police officers react to a second explosion at the finish line of the Boston Marathon. (April 15, 2013) (Credit: AP )

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A man comforts an injured woman on the
A man comforts an injured woman on the sidewalk after the first explosion on Boylston Street, near the finish line of the Boston Marathon. (April 15, 2013) (Credit: Boston Globe)
Runner John Ounao cries when he finds friends
Runner John Ounao cries when he finds friends after explosions rocked the finish of the Boston Marathon in Massachusetts. (April 15, 2013) (Credit: Getty Images)
Boston Firefighter James Plourde carries an injured girl
Boston Firefighter James Plourde carries an injured girl away from the scene after a bombing near the finish line of the Boston Marathon. (April 15, 2013) (Credit: AP)
A runner embraces another woman on the marathon
A runner embraces another woman on the marathon route near Kenmore Square after two bombs exploded during the 117th Boston Marathon. (April 15, 2013) (Credit: Getty Images)
People gathered at the Boston Marathon react as
People gathered at the Boston Marathon react as one of two explosions goes off near the finish line. (April 15, 2013) (Credit: AP)

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Authorities investigate one of the blast sites on
Authorities investigate one of the blast sites on Boylston Street near the finish line of the 2013 Boston Marathon. (April 15, 2013) (Credit: AP)
A Boston Marathon competitor and Boston police run
A Boston Marathon competitor and Boston police run from the area of an explosion near the finish line. (April 15, 2013) (Credit: AP )
People react to an explosion at the 2013
People react to an explosion at the 2013 Boston Marathon. Two explosions shattered the euphoria at the finish line, sending authorities out on the course to carry off the injured while stragglers were rerouted away from the smoking site of the blasts. (April 15, 2013) (Credit: AP)
Rescue personnel respond to the scene near the
Rescue personnel respond to the scene near the finish line after explosions at the 2013 Boston Marathon. (April 15, 2013) (Credit: AP )
A runner reacts near the explosions during
A runner reacts near the explosions during the Boston Marathon. (April 15, 2013) (Credit: Getty Images)
Boston Marathon runner Russell Clifford of Marlborough, Mass.,
Boston Marathon runner Russell Clifford of Marlborough, Mass., walks past SWAT officers near the finish line where explosions killed and injured both runners and spectators. (April 15, 2013) (Credit: AP)
Trash remains on the street near the finish
Trash remains on the street near the finish line of the Boston Marathon, where several explosions rocked the event, killing several and wounding more than 140. (April 15, 2013) (Credit: Getty Images)
Rescue workers and spectators help a victim of
Rescue workers and spectators help a victim of the bombings at the Boston Marathon finish line after the explosion. (April 15, 2013) (Credit: AP)
Victims of the bomb blasts at the Boston
Victims of the bomb blasts at the Boston Marathon are helped from the scene to where they will receive medical attention. (April 15, 2013) (Credit: MCT)
Staff gather outside a medical tent near the
Staff gather outside a medical tent near the finish line of the Boston Marathon, where injured participants and spectators were brought after multiple explosions rocked the event. (April 15, 2013) (Credit: Getty Images)
An injured woman is tended to at the
An injured woman is tended to at the finish line of the Boston Marathon after explosions shattered the euphoria of the event. (April 15, 2013) (Credit: AP)
Medical workers aid an injured spectator at the
Medical workers aid an injured spectator at the 2013 Boston Marathon after explosions near the finish line of the race. (April 15, 2013) (Credit: AP)
Medical workers help an injured man at the
Medical workers help an injured man at the 2013 Boston Marathon after two bombs exploded near the finish line of the race, injuring more than 140 spectators and participants. (April 15, 2013) (Credit: AP)
Pedestrians walk down Boylston Street after two explosions
Pedestrians walk down Boylston Street after two explosions occurred along the final stretch of the Boston Marathon. (April 15, 2013) (Credit: Bloomberg)
A runner and a woman embrace on Boston
A runner and a woman embrace on Boston Common after two explosions occurred along the final stretch of the Boston Marathon on Boylston Street. (April 15, 2013) (Credit: Bloomberg)
An unidentified woman is escorted by DEA agents
An unidentified woman is escorted by DEA agents from the main entrance to Brigham and Women's Hospital in Boston, where many wounded from two explosions near the finish line of the Boston Marathon were brought. (April 15, 2013) (Credit: Getty Images)
Officials from the Boston Fire Department Hazardous Materials
Officials from the Boston Fire Department Hazardous Materials Unit wrap up a garbage can along Commonwealth Avenue after two explosions occurred along the final stretch of the Boston Marathon on Boylston Street. (April 15, 2013) (Credit: Bloomberg)
A runner retrieves his belongings from the sorting
A runner retrieves his belongings from the sorting area after explosions at the finish line put a premature end to the Boston Marathon. (April 15, 2013) (Credit: AP)
A woman in running clothes leaves the main
A woman in running clothes leaves the main entrance of Bringham and Women's Hospital in Boston, where many victims were brought after explosions near the finish line of the Boston Marathon. (April 15, 2013) (Credit: Getty Images)
Boston SWAT members leave the Boston Common staging
Boston SWAT members leave the Boston Common staging area at the Boston Marathon where several explosions rocked the event, sending more than 140 people to local hospitals. (April 15, 2013) (Credit: Getty Images)
View of the back side of the finish
View of the back side of the finish line at the Boston Marathon, where several explosions rocked the event and sent more than 140 people to local hospitals. (April 15, 2013) (Credit: Getty Images)
A SWAT team arrives at the corner of
A SWAT team arrives at the corner of Stuart and Dartmouth streets after two explosive devices detonated near the finish line of the 117th Boston Marathon. (April 15, 2013) (Credit: Getty Images)
Military personnel gather at the Boston Common staging
Military personnel gather at the Boston Common staging at the Boston Marathon after explosions thought to be part of an organized terror attack rocked the event. (April 15, 2013) (Credit: Getty Images)
A member of the bomb squad investigates a
A member of the bomb squad investigates a suspicious item on the road near Kenmore Square after two bombs exploded during the 117th Boston Marathon. (April 15, 2013) (Credit: Getty Images)
People walk near Kenmore Square after two bombs
People walk near Kenmore Square after two bombs exploded during the 117th Boston Marathon in Massachusetts. (April 15, 2013) (Credit: Getty Images)
A Boston police officer patrols the area near
A Boston police officer patrols the area near the finish line after the explosions at the 2013 Boston Marathon. (April 15, 2013) (Credit: AP )
Injured people lie on the sidewalk near the
Injured people lie on the sidewalk near the Boston Marathon finish line after an explosion. (April 15, 2013) (Credit: AP)
An armed FBI agent passes a Boston police
An armed FBI agent passes a Boston police officer following explosions at the finish line of the 2013 Boston Marathon. (April 15, 2013) (Credit: AP)
Emergency responders gather around the finish line of
Emergency responders gather around the finish line of the Boston Marathon following explosions that sent more than 140 people to the hospital. (April 15, 2013) (Credit: AP )
Armed Massachusetts State Police roll into the area
Armed Massachusetts State Police roll into the area following an explosion at the 2013 Boston Marathon in Boston. (April 15, 2013) (Credit: AP )
Firefighters take postion on Boyltson Street near the
Firefighters take postion on Boyltson Street near the finish line after two bombs exploded during the 117th Boston Marathon in Boston, Massachusetts. (April 15, 2013) (Credit: Getty Images )
State police guard an area near Kenmore Square
State police guard an area near Kenmore Square after two bombs exploded during the 117th Boston Marathon in Boston, Massachusetts. (April 15, 2013) (Credit: Getty Images)
President Barack Obama talks on the phone with
President Barack Obama talks on the phone with FBI Director Robert Mueller to receive an update on the explosions that occurred in Boston, in the Oval Office. Seated with him are Lisa Monaco, Assistant to the President for Homeland Security and Counterterrorism, and Chief of Staff Denis McDonough. (April 15, 2013) (Credit: White House)
Medical personnel work outside the medical tent in
Medical personnel work outside the medical tent in the aftermath of two blasts which exploded near the finish line of the Boston Marathon in Boston. (April 15, 2013) (Credit: AP)
Boston police clear an area following an explosion
Boston police clear an area following an explosion near the finish line of the 2013 Boston Marathon in Boston. (April 15, 2013) (Credit: AP )
Two Boston Marathon volunteers hug each other, at
Two Boston Marathon volunteers hug each other, at right, as an injured person is loaded into an ambulance in the aftermath of two blasts which exploded near the finish line of the Boston Marathon in Boston. (April 15, 2013) (Credit: AP)
A man is loaded into an ambulance after
A man is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square in Boston, Massachusetts. (April 15, 2013) (Credit: Getty Images)
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