Authorities recover pieces of one of the explosive devices

Authorities investigating the deadly bombings at the Boston Marathon have found part of a pressure cooker used in the attack.

It comes after the recovery on Tuesday night of part of a circuit board they believe was part of one of the devices.

Officials said the lid of a pressure cooker was apparently catapulted onto the roof of a nearby building.

A person close to the investigation previously said the bombs consisted of explosives put in six-litre pressure cookers, one with shards of metal and ball bearings, the other with nails.

Law enforcement agencies pleaded for the public to come forward with photos, videos, or any information that might help them solve the twin bombings at the finish line which killed three people and wounded more than 170.

Investigators have circulated information about the bombs but the FBI said that nobody had claimed responsibility.

“Someone knows who did this,” said FBI agent Richard DesLauriers.

“Importantly, the person who did this is someone’s friend, neighbour, co-worker, or relative.”

President Barack Obama branded the attack an act of terrorism but said officials did not know “whether it was planned and executed by a terrorist organisation, foreign or domestic, or was the act of a malevolent ind-ividual”.

Obama plans to attend an interfaith service today in the victims’ honour in Boston.

At Massachusetts General Hospital, all four amputations performed there were above the knee, with no hope of saving more of the legs, said Dr George Velmahos, chief of trauma surgery.

“It wasn’t a hard decision to make,” he said.

“We just completed the ugly job that the bomb did.”

An intelligence bulletin issued to law enforcement includes a picture of a mangled pressure cooker and a torn black bag that the FBI said were part of a bomb that exploded during the marathon.

DesLauriers said co-operation from the community will play a key role in the investigation. He said the range of suspects remained wide open, but more than 2,000 tips had been received.

The bombs exploded 10 or more seconds apart, tearing off victims’ limbs and spattering streets with blood.

The blasts near the finish line instantly turned the festive race into a hellish scene of confusion, horror and heroics.

Officials found that the bombs in Boston consisted of explosives put in ordinary, six-litre pressure cookers, one with shards of metal and ball bearings.

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

Investigators found pieces of black nylon from a bag or backpack and fragments nails, possibly contained in a pressure cooker.