FBI agents zeroed in Tuesday on how the Boston Marathon bombing was carried out with kitchen pressure-cookers packed with explosives, nails and other lethal shrapnel but said they still didn’t know who did it and why.

An intelligence bulletin issued to law enforcement and released late Tuesday includes a picture of a mangled pressure-cooker and a torn black bag the FBI said were part of a bomb.

The FBI and other prominent law enforcement agencies repeatedly pleaded for members of the public to come forward with photos, videos or anything suspicious they might have seen or heard.

President Barack Obama branded the attack an act of terrorism but said officials don’t know “whether it was planned and executed by a terrorist organisation, foreign or domestic, or was the act of a malevolent individual.”

Richard DesLauriers, FBI agent in charge in Boston, said at a news conference that the “range of suspects and motives remains wide open.” He vowed to “go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime.”

Scores of victims of the Boston bombing remained in hospitals, many with grievous injuries, a day after the twin explosions near the marathon’s finish line killed three people, wounded more than 170 and reawakened fears of terrorism.

Officials found that the bombs in Boston consisted of explosives put in ordinary, 6-litre pressure-cookers, one with shards of metal and ball bearings, the other with nails, according to a person close to the investigation who spoke on condition of anonymity.

Both bombs were stuffed into black duffel bags and left on the ground, the person said.

Mr. 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, Virginia, 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,” said Jason Pack, FBI spokesman in Boston.

Investigators said they have not yet determined what was used to set off the explosives.

Mr. DesLauriers said there had been no claim of responsibility for the attack.

“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, Massachusetts, and a third victim, identified only as a graduate student at Boston University.

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