The FBI has released images from the probe into the deadly attack on the Boston Marathon showing details of the bombs used.
Images from a joint Homeland Security and FBI bulletin show the remains of a dark colored backpack and remnants of what appears to be a pressure cooker.
Three people died, including an 8-year-old boy, and more than 180 were injured when two bombs exploded near the finish line of Monday’s race.
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.
Richard DesLauriers added: “The investigation is in its infancy. There are no claims of responsibility and the range of suspects and motives remains wide open.”
He 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.
It has also been reported that a circuit board and battery pack – parts of a triggering mechanism – were also recovered.
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.
Speaking at the same news conference, Massachusetts Governor Deval Patrick said the Boston community would “recover and heal” from the attack.
“This is one community. We are all in this together,” he said.
President Barack Obama will address an interfaith service in Boston for the victims on Thursday morning.
The White House said Barack Obama had cancelled a planned trip to Kansas to speak.