On their own, they're simple household items.
Pressure cookers. Nails. Nylon bags.
But combine them with explosives, and you can get the kind of bombs that investigators now suspect were used to turn the finish line of the Boston Marathon into a bloody nightmare.
Scraps of them found at the scene of the twin bombings are being sent to the FBI's national laboratory, where technicians will try to reconstruct the devices, the federal agent leading the investigation said Tuesday. Meanwhile, investigators combed through a digital mountain of photos and video clips and asked the public for help finding who planted them at two points on Boylston Street.
"Someone knows who did this," said Rick DesLauriers, the special agent in charge of the FBI's Boston office. "Cooperation from the community will play a crucial role in this investigation."
DesLauriers asked people to report anyone who talked about targeting Monday's race or showed interest in explosives. He urged anyone who might have heard the sounds of explosions in remote areas -- possibly by someone testing a bomb -- or saw someone carrying "an unusually heavy, dark-colored bag" around Monday to come forward.
There has been no claim of responsibility, DesLauriers said. "The range of suspects and motives remains wide open," he said.
The U.S. government has warned federal agencies in the past that pressure cookers -- airtight pots used to quickly cook or preserve foods -- have been packed with explosives and shrapnel and detonated with blasting camps. A law enforcement official said Monday's bombs were likely detonated by timers.
Photos obtained by CNN showed the remains of a pressure cooker found at the scene, along with a shredded black backpack and what appear to be metal pellets or ball bearings. A federal law enforcement source told CNN the photos were included in a bulletin sent to federal law enforcement agencies.
The pieces recovered so far suggest the devices, which were identical, could carry 6 liters (1.5 gallons) of liquid apiece, a Boston law enforcement source said. The parts found include a partial circuit board, which would be used to detonate a device.
'Very effective' bombs
Pressure cooker bombs can be "very effective," terrorism expert Jeff Beatty told CNN. Taliban and al Qaeda militants "use them to make their IEDs," he said.
"That doesn't mean it was the Taliban -- other people can read about this," said Beatty, who served in the FBI, CIA and the military's Delta Force.
But so far, investigators have found no foreign or al Qaeda connection to the bombings, a U.S. official told CNN. And Sen. Saxby Chambliss, the ranking Republican on the Senate Intelligence Committee, told reporters in Washington that some of the facts suggest the carnage had its roots in American soil.
"There are a lot of things that are surrounding this that would give an indication that it may have been a domestic terrorist, but that just can't be assumed," Chambliss said.
Homeland Security Secretary Janet Napolitano said the bombings don't appear to be harbingers of "a broader plot." But she urged Americans "to remain vigilant and immediately report any signs of suspicious activity to local law enforcement officials."
And members of Congress who were briefed by Obama administration officials on told reporters that the field of possible suspects remains wide open.
"We really don't know if it's a foreign or domestic threat," said Rep. Michael McCaul, R-Texas, chairman of the House Homeland Security Commitee. "We don't know whether this was a homegrown terrorist or part of a wider conspiracy."
Director of National Intelligence James Clapper held closed-door talks with the Senate Intelligence Committee on Tuesday. Chambliss,R-Georgia, told reporters afterward that "there's not a whole lot of update to give."
"This is a very fluid investigation. The FBI's in the lead, and I personally know the special agent in charge. He's one of the best," Chambliss said.
Late Monday, authorities searched the apartment of a young Saudi man who was among the wounded at the race. He was found to have no connection to the attack.
"He was just at the wrong place at the wrong time," the U.S. official said.
A Saudi woman, a medical student who was also injured in the blast, has also been interviewed by investigators, according to a law enforcement source.
Boston's 'most complex crime scene' ever
Investigators are combing what Boston Police Commissioner Ed Davis called "the most complex crime scene that we've dealt with in the history of our department." The cordoned off area has been reduced from 15 blocks to 12 and will be reduced further in coming days, he said.