DesLauriers confirmed that investigators had found pieces of black nylon from a bag or backpack and fragments of BBs and nails, possibly contained in a pressure cooker. He said the items were sent to the FBI laboratory at Quantico, Va., for analysis.
The FBI said it is looking at what Boston television station WHDH said are photos sent by a viewer that show the scene right before and after the bombs went off. The photo shows something next to a mailbox that appears to be a bag, but it’s unclear what the significance is.
“We’re taking a look at hundreds of photos, and that’s one of them,” FBI spokesman Jason Pack said.
Investigators said they haven’t determined what was used to set off the explosives.
Pressure cooker explosives have been used in international terrorism and have been recommended for lone-wolf operatives by al-Qaida’s branch in Yemen.
But information on how to make the bombs is readily found online, and U.S. officials said Americans should not rush to judgment in linking the attack to overseas terrorists.
DesLauriers said there had been no claim of responsibility for the attack.
He urged people to come forward with anything suspicious, such as hearing someone express an interest in explosives or a desire to attack the marathon, seeing someone carrying a dark heavy bag at the race or hearing mysterious explosions recently.
“Someone knows who did this,” the FBI agent said.
The bombs exploded 10 or more seconds apart, tearing off victims’ limbs and spattering streets with blood, instantly turning the festive race into a hellish scene of confusion, horror and heroics.
The blasts killed 8-year-old Martin Richard, of Boston, 29-year-old Krystle Campbell, of Medford, and a third victim, identified only as a graduate student at Boston University.