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.
While the clues moved the investigation forward, they did not make it immediately apparent 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."
Third victim identified
Boston University identified graduate student Lingzu Lu as the third person who died in Monday's bombings.
Previously identified were Krystle Campbell, 29, of Arlington, Massachusetts, and Martin Richard, 8, of Dorchester, Massachusetts.
"She was the best," Campbell's distraught mother, Patty, told reporters Tuesday. "You couldn't ask for a better daughter."
Martin "was a bright, energetic young boy who had big dreams and high hopes for his future," his school said in a statement. "We are heartbroken by this loss."
The hunt for the attacker
The attack 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 sifted through thousands of pieces of evidence and a mass of digital photos and video clips. They have pleaded 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, said. "The community will play a crucial role in this investigation."
Medical personnel treating the wounded found evidence suggesting the bomb maker or bomb makers sought to maximize the suffering.
Dr. George Velmahos, head of trauma care at Massachusetts General Hospital, said his team found "numerous" metal pellets and nails inside patients' bodies.
"There are people who have 10, 20, 30, 40 of them in their body, or more," Velmahos said.
Dr. Ron Walls also said one patient had more than 12 carpenter-type nails.
"There is no question some of these objects were implanted in the device for the purpose of being exploded forward," he said.
Victims continue recovery