Investigators believe they have identified a suspect in the Boston Marathon bombings, a source who has been briefed on the investigation told CNN's John King exclusively.
The breakthrough came from analysis of video from a department store near the site of the second explosion. Video from a Boston television station also contributed to the progress, said the source, who declined to be more specific but called it a significant development.
Earlier, a federal law enforcement source with firsthand knowledge of the investigation told CNN that a lid to a pressure cooker thought to have been used in the bombings had been found on a roof of a building near the scene.
While such clues may move the investigation forward, they did not reveal whether the attack was an act of domestic or foreign terrorism.
"If your experience and your expertise is Middle East terrorism, it has the hallmarks of al Qaeda or a Middle East group," former FBI Assistant Director Tom Fuentes said. "If your experience is domestic groups and bombings that have occurred here, it has the hallmarks of a domestic terrorist like Eric Rudolph in the 1996 Atlanta Summer Olympics bombings."
Fuentes said he has investigated both types of terrorism -- from Iraq to the United States -- and finds the Boston attack has elements of both. "It has the hallmarks of both domestic and international (attacks), and you can see either side of that."
The bombs, which exploded 12 seconds apart, killed three people and wounded 183.
One was housed in a pressure cooker hidden inside a backpack, the FBI said in a joint intelligence bulletin. The device also had fragments that may have included nails, BBs and ball bearings, the agency said.
The second bomb was also housed in a metal container, but it was not clear whether it too was in a pressure cooker, the FBI said.
The U.S. government has warned federal agencies in the past that terrorists could turn pressure cookers into bombs by packing them with explosives and shrapnel and detonating them with blasting caps.
Photos obtained by CNN show 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.
Scraps of at least one pressure cooker, nails and nylon bags found at the scene were sent to the FBI's national laboratory in Virginia, where technicians will try to reconstruct the devices, the agent leading the investigation said Tuesday.
The pieces suggest each of the devices was 6 liters (about 1.6 gallons) in volume, a Boston law enforcement source said. The recovered parts include part of a circuit board, which might have been used to detonate a device.
A law enforcement official said Monday's bombs were probably detonated by timers. But the FBI said details of the detonating system were unknown.
The hunt for the attacker
The attack has left Boston police with "the most complex crime scene that we've dealt with in the history of our department," Commissioner Ed Davis said Tuesday.
Authorities are sifting through more than 2,000 tips and a mass of digital photos and video clips, but are asking for the public's help in providing additional leads and images.
"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 the public to report anyone who may have talked about targeting the marathon or shown interest in explosives. He urged anyone who may have heard explosions in remote areas -- possibly indicating a bomb test -- or seen someone carrying "an unusually heavy, dark-colored bag" around the time of the attack to come forward.
Who may have wanted to cause mass destruction -- and whether the individual or individuals acted alone or as part of a group -- remained a mystery.
"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 Committee. "We don't know whether this was a homegrown terrorist or part of a wider conspiracy."
A "lone wolf" attacker could be particularly difficult to identify.
"This is what you worry about the most," a source with knowledge of the investigation said. "No trail, no intelligence."
"Boston is not going to be intimidated by this," said U.S. Secretary of State John Kerry, a former U.S. senator from Massachusetts, in testimony to the House Foreign Affairs Committee on Wednesday. "But we are going to find out who did this. The police work being done is extraordinary. The FBI is remarkable. There is a great deal of forensic evidence. We are hopeful we can bring people to justice."