Bombs that targeted the Boston Marathon on Monday may have been packed in pressure cookers, investigators say.

调查人员称，周一波士顿马拉松爆炸案中所用的炸弹可能被藏于高压锅中。

Images from a joint Homeland Security and FBI bulletin show the remains of a dark backpack, a detonation device and mangled pieces of metal.

Three people were killed and more than 170 injured when two bombs exploded near the finish line of Monday's race.

Those who died were an eight-year-old boy, a woman aged 29 and a postgraduate student from China.

The BBC's Paul Adams, in Boston, says vigils（守夜，监视） for the victims were held across the city on Tuesday night as Bostonians still wrestled with why anyone would want to attack their much-loved marathon.

President Barack Obama will travel to Boston on Thursday for a memorial.

'Someone knows'

FBI Special Agent Richard DesLauriers told a news conference that pieces of nylon had been recovered from the scene, along with fragments of ball bearings and nails that were "possibly contained in a pressure-cooker device".

He said they were being sent to the agency's laboratory in Quantico, Virginia, where experts would reconstruct the devices to determine their make-up and components.

He added: "The investigation is in its infancy. There are no claims of responsibility and the range of suspects and motives remains wide open."

Mr DesLauriers urged people to report anyone they had seen acting suspiciously.

"Someone knows who did this," he said.

Associated Press quoted a source close to the investigation as saying that the bombs consisted of explosives placed in 1.6-gallon pressure cookers, one with shards of metal and ball bearings, the other with nails.

The bombs were put into black bags and left on the ground, the source said.

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