Even the smallest bits of debris could help indicate the bombs' "signature," said a federal law enforcement official who works in the intelligence community. The explosives themselves were small, and initial tests showed no C-4 or other high-grade explosive material -- suggesting the packages used in the attack were crude devices.
Based on the bombs' effects, the devices could have been small enough to be concealed in small bags or boxes, a law enforcement official said. The smoke was consistent with a "low-velocity improvised explosive mixture, perhaps flash powder or sugar chlorate mixture," the official said.
Two photos obtained by CNN affiliate WHDH might raise more questions. The first shows a light-colored bag sitting on the ground next to a mailbox. The second -- of the same spot -- shows a blast seemingly where that bag sat, with the mailbox still upright. Investigators have not commented on these photos, which WHDH reports it has shared with authorities.
When the pieces that have been recovered are sent to the FBI laboratory in Quantico, Virginia, investigators will try to better understand how they worked and look for clues like fingerprints, DNA or serial numbers to help track who planted them, the Boston law enforcement source said.
Authorities haven't given any timetable as to when they expect significantly new details. But DesLauriers vowed Tuesday that the more than 1,000 law enforcement officers from more than 30 agencies will get results, even if they have to go "to the ends of the Earth" to get them.
"Our mission is clear: to bring to justice those responsible," he said. "The American public wants answers. The citizens of the city of Boston and the Commonwealth of Massachusetts want and deserve answers."
Authorities have begun to search through huge amounts of video and images from surveillance cameras in the area near the attack. So far, no footage has been spotted showing someone placing the bombs, a law enforcement source said.
Authorities have asked anyone with images from any part of the marathon to share them with police.
"People don't know that they were witnesses -- that they might actually have evidence in their phones or in their cameras," Juliette Kayyem, President Obama's former assistant secretary for homeland security, said on CNN's "Starting Point."
Davis vowed authorities will sift "through every frame of every video."
The FBI is likely issuing subpoenas for records from cell towers in the area to isolate and trace calls from around Copley Square at the time of the blasts, according to a federal law enforcement official.
Doctors believe bombs contained sharp objects
Two doctors overseeing treatment of the injured believe the explosive devices contained nails or similar objects.
Many patients have severe wounds "related to the blast effect of the bomb as well as small metallic fragments that entered their body," including "pellets" and "nail-like objects," said Dr. George Velmahos, head of trauma care at Massachusetts General Hospital.
A variety of sharp objects were found inside the patients' bodies, he said, adding that the bombs probably contained multiple metallic fragments.
Asked whether what was found in the patients' bodies could have come from nearby objects that exploded in the blast, Velmahos said he believes the materials were likely part of the explosive devices.
Ron Walls, chair of emergency medicine at Brigham and Women's Hospital, said most patients there were wounded by "ordinary debris." But three were injured by "perfectly round objects" that were "very uniform, consistent, metallic," he said. And another 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," Wall said.
No unexploded bombs
Suspicious packages that were detonated out of precaution after the bombings turned out not to be explosive devices after all.
After the blasts Monday, some officials reported that explosive devices that failed to go off were found.
But investigators said Tuesday the only bombs were the two that exploded at the marathon.
The intelligence community is poring through all threat reporting for any clues, U.S. counterterrorism officials told CNN.
That includes any claims made on jihadist websites. Nothing is being dismissed this early on, the officials said.