CNN and other news outlets reported but have now retracted information that an arrest has been made in the Boston bombings.
The FBI has also said no arrest has been made.
Everyone seems agreed a suspect has been identified.
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.