In kitchens, they prepare food faster, but pressure cookers by their very nature help make good bombs, amplifying the blast and the carnage.

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

Boston Marathon investigators have found a piece of circuit board they believe was part of one of the bombs.

FBI agents zeroed in 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 did not know who did it and why.

Officials investigating the Boston Marathon bombing said no additional explosive devices have been discovered other than the two that detonated near the race’s finish line, a development that could complicate the case.

The Boston marathon bombs were explosives packed in 6-litre pressure cookers and placed in black duffel bags, according to a source close to the investigation.

US President Barack Obama has said they do not know who carried out the Boston marathon bombings or why.