Boston Marathon bombing

Gallery: A woman visits a makeshift memorial on a blocked-off section of Boylston Street in Boston, April 17, 2013. Officials said they were still at the start of an inquiry on the attack at the Boston Marathon that killed three people and hurt more than 170 Monday, using what were likely kitchen pressure cookers set up to shoot shrapnel.

Gallery: Investigators comb the site of the second of two bomb explosions on Boylston Street in Boston, April 17, 2013. Officials said they were still at the start of an inquiry on the attack at the Boston Marathon that killed three people and hurt more than 170 Monday, using what were likely kitchen pressure cookers set up to shoot shrapnel.

Gallery: Police and investigators scour Boylston Street, near the finish line of Monday's Boston Marathon, in Boston, Wednesday, April 17, 2013. Authorities investigating the deadly bombings at the Boston Marathon have recovered a piece of circuit board that they believe was part of one of the explosive devices, and also found the lid of a pressure cooker that apparently was catapulted onto the roof of a nearby building, an official said Wednesday. (AP Photo/Charles Krupa)

Gallery: Boston University student Joy Liu places a note at a memorial on the campus for Boston Marathon bombing victim Lu Lingzi in Boston Wednesday, April 17, 2013. Lu and two friends had been watching the Boston Marathon near the finish line.

Gallery: In this handout photo released by the Joint Terrorism Task Force of Boston, the scene at one of the blast sites in Boston. Law enforcement officials said Tuesday April 16, 2013 that the explosives that killed three people and injured more than 170 during the Boston Marathon on Monday were most likely rudimentary devices made from ordinary kitchen pressure cookers. (Joint Terrorism Task Force of Boston via The New York Times) -- EDITORIAL USE ONLY

Gallery: The site of the first of two bomb explosions on Boylston Street in Boston, April 17, 2013. Officials said they were still at the start of an inquiry on the attack at the Boston Marathon that killed three people and hurt more than 170 Monday, using what were likely kitchen pressure cookers set up to shoot shrapnel. (Eric Thayer/The New York Times)

Gallery: In this undated handout photo released by the Joint Terrorism Task Force of Boston, a piece of what the FBI says was a pressure cooker that held one of the bombs that exploded during the Boston Marathon. Law enforcement officials said April 16, that the explosives that killed three people and injured more than 170 during the Boston Marathon on Monday were most likely rudimentary devices made from ordinary kitchen pressure cookers. (Joint Terrorism Task Force of Boston via The New York Times) -- EDITORIAL USE ONLY

Gallery: In this handout photo released by the Joint Terrorism Task Force of Boston, a piece found Tuesday, April 16, 2013, of what officials said may have been part of the devices used during the Boston Marathon bombings. Law enforcement officials said April 16, that the explosives that killed three people and injured more than 170 during the Boston Marathon on Monday were most likely rudimentary devices made from ordinary kitchen pressure cookers. (Joint Terrorism Task Force of Boston via The New York Times) -- EDITORIAL USE ONLY

Gallery: In this undated handout photo released by the Joint Terrorism Task Force of Boston, shrapnel and a remnant of what appears to be the explosive device used during the Boston Marathon bombings is measured. Law enforcement officials said Tuesday, April 16, 2013, that the explosives that killed three people and injured more than 170 during the Boston Marathon on Monday were most likely rudimentary devices made from ordinary kitchen pressure cookers. (Joint Terrorism Task Force of Boston via The New York Times) -- EDITORIAL USE ONLY

Gallery: In this undated handout photo released by the Joint Terrorism Task Force of Boston, a fragment of what appears to be the explosive device used during the Boston Marathon bombings. Law enforcement officials said Tuesday, April 16, 2013, that the explosives that killed three people and injured more than 170 during the Boston Marathon on Monday were most likely rudimentary devices made from ordinary kitchen pressure cookers. (Joint Terrorism Task Force of Boston via The New York Times) -- EDITORIAL USE ONLY

Gallery: In this undated handout photo released by the FBI, a shredded black backpack that the bureau says contained one of the bombs that exploded during the Boston Marathon. Law enforcement officials said Tuesday, April 16, 2013, that the explosives that killed three people and injured more than 170 during the Boston Marathon on Monday were most likely rudimentary devices made from ordinary kitchen pressure cookers. (FBI via The New York Times) -- EDITORIAL USE ONLY

Gallery: In this undated handout photo released by the FBI, a piece of what the bureau says was a pressure cooker that held one of the bombs that exploded during the Boston Marathon. Law enforcement officials said Tuesday, April 16, 2013, that the explosives that killed three people and injured more than 170 during the Boston Marathon on Monday were most likely rudimentary devices made from ordinary kitchen pressure cookers. (FBI via The New York Times) -- EDITORIAL USE ONLY

Gallery: Military personnel near Boylston Street in Boston, April 17, 2013. Officials said they were still at the start of an inquiry on the attack at the Boston Marathon that killed three people and hurt more than 170 Monday, using what were likely kitchen pressure cookers set up to shoot shrapnel. (Katherine Taylor/The New York Times)

Gallery: Police officer Gary Beaudoin and his bomb-sniffing dog, Xtra, on Arlington Street in Boston, April 17, 2013. Officials said they were still at the start of an inquiry on the attack at the Boston Marathon that killed three people and hurt more than 170 Monday, using what were likely kitchen pressure cookers set up to shoot shrapnel.

Gallery: Mary-Ann Knott-Craig, of Watertown, Mass., right, and Dawn Sommerville, of the South Boston neighborhood, pause at the site of a makeshift memorial near the finish line of Monday's Boston Marathon in Boston, Wednesday, April 17, 2013. The bombs that blew up seconds apart near the finish line left the streets spattered with blood and glass, and gaping questions of who chose to attack at the Boston Marathon and why.

Gallery: A man with a jacket from the 2013 Boston Marathon visits a makeshift memorial at the corner of Boylston and Berkeley Streets in Boston, April 17, 2013. A lid from one of the pressure cookers that investigators believe were used in the bombings at the Boston Marathon on Monday was found on a rooftop, giving a sense of the force of the blast.