Media playback is unsupported on your device Media caption The bombs apparently involved pressure cookers stuffed with metal shards

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

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 devices 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 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.

"The investigation is in its infancy," he added. "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.

The Associated Press quoted a source close to the investigation as saying that the bombs consisted of explosives placed in 1.6-gallon (6-litre) 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.

It has been reported that a circuit board and battery pack - parts of a triggering mechanism - were recovered.

Media playback is unsupported on your device Media caption FBI Special Agent Richard DesLauriers gives details of possible bomb components

And the lid of a pressure cooker, apparently blown off during the explosion, was found on the roof of a nearby building, an official said on Wednesday.

Doctors treating the wounded say their injuries indicate that the bombs contained metal shards and other shrapnel. A number of victims have had limbs amputated.

Boston Medical Center trauma surgery chief Peter Burke said hospitals were saving "large quantities" of fragments extracted from victims for the police. They include metal, plastic, wood and concrete.

At least 58 of the injured have been released from various hospitals around the city, according to AP. Of those that remain, 17 patients are listed as in a critical condition.

President Obama will address an interfaith service in Boston for the victims on Thursday morning.

The White House said Mr Obama had cancelled a planned trip to Kansas to speak.

President Obama has condemned the bombings as a terrorist act.

Image caption Pressure cookers are cheap and easy to buy. Details of a pressure cooker bomb were once published in an online al-Qaeda magazine. It was one of three devices used in the 2010 attempted Times Square attack.

Image caption The US Dept of Homeland Security has issued two previous warnings about the dangers of pressure cooker bombs. The cooker is a tightly-sealed pot, which can increase the force of the blast when packed with explosives.

Image caption The explosion can be triggered using a simple device like a digital watch or mobile phone.