Story highlights Remains of a pressure cooker found at scene, official says

FBI technicians will attempt to reconstruct the Boston bombs

Scraps of metal and black nylon were found at the scene

"The range of suspects and motives remains wide open," FBI agent says

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.

Photos: Deadly attack at Boston Marathon Photos: Deadly attack at Boston Marathon An injured man is loaded into an ambulance after two bombs went off near the finish line of the Boston Marathon on April 15, 2013. Three people were killed and at least 264 were injured. Hide Caption 1 of 55 Photos: Deadly attack at Boston Marathon A man comforts a victim on the sidewalk at the scene of the first explosion. Hide Caption 2 of 55 Photos: Deadly attack at Boston Marathon The second explosion goes off near the finish line. Hide Caption 3 of 55 Photos: Deadly attack at Boston Marathon The first explosion knocked down 78-year-old runner Bill Iffrig at the finish line. He got up a few minutes later and finished the race. Hide Caption 4 of 55 Photos: Deadly attack at Boston Marathon A runner embraces another woman on the marathon route near Kenmore Square. Hide Caption 5 of 55 Photos: Deadly attack at Boston Marathon Former New England Patriots offensive lineman Joe Andruzzi carries a woman from the scene. Hide Caption 6 of 55 Photos: Deadly attack at Boston Marathon A victim of the first explosion is helped on the sidewalk of Boylston Street. Hide Caption 7 of 55 Photos: Deadly attack at Boston Marathon An injured person is taken away from the scene in a wheelchair. Hide Caption 8 of 55 Photos: Deadly attack at Boston Marathon A woman kneels and prays at near the finish line. Hide Caption 9 of 55 Photos: Deadly attack at Boston Marathon People run down Exeter Street after the blasts. Hide Caption 10 of 55 Photos: Deadly attack at Boston Marathon The explosions occurred around 2:45 p.m., about an hour after the first of the race's nearly 27,000 runners had crossed the finish line. Hide Caption 11 of 55 Photos: Deadly attack at Boston Marathon Victims lie on the ground at the scene of the first explosion. Hide Caption 12 of 55 Photos: Deadly attack at Boston Marathon A couple runs from the scene pushing a stroller. Hide Caption 13 of 55 Photos: Deadly attack at Boston Marathon A runner reacts near Kenmore Square after the explosions. Hide Caption 14 of 55 Photos: Deadly attack at Boston Marathon A man's blood-stained feet hang outside an ambulance. Hide Caption 15 of 55 Photos: Deadly attack at Boston Marathon A bystander who was injured in the first explosion is wheeled across the finish line while receiving medical attention from rescue workers. Hide Caption 16 of 55 Photos: Deadly attack at Boston Marathon Carlos Arredondo was at the race handing out American flags to spectators. After the blasts, he helped emergency responders and is credited with helping a man survive serious leg wounds. Hide Caption 17 of 55 Photos: Deadly attack at Boston Marathon The second explosion goes off near the finish line. Hide Caption 18 of 55 Photos: Deadly attack at Boston Marathon Police look at blown-out windows. The bombs shook buildings, witnesses said, sending people to seek shelter under tables. Hide Caption 19 of 55 Photos: Deadly attack at Boston Marathon Unclaimed runners' bags fill an area near the marathon finish. Hide Caption 20 of 55 Photos: Deadly attack at Boston Marathon Women desperate to hear from loved ones are unable to get close to the site of the attack. Hide Caption 21 of 55 Photos: Deadly attack at Boston Marathon Police and runners stand near Kenmore Square after the attack. Hide Caption 22 of 55 Photos: Deadly attack at Boston Marathon Runners gather near Kenmore Square after the explosions. Hide Caption 23 of 55 Photos: Deadly attack at Boston Marathon An EMT worker is transferred to an ambulance outside a medical tent in Copley Square. Hide Caption 24 of 55 Photos: Deadly attack at Boston Marathon A SWAT team arrives on the scene. Hide Caption 25 of 55 Photos: Deadly attack at Boston Marathon A runner sits near Kenmore Square after the attack. Hide Caption 26 of 55 Photos: Deadly attack at Boston Marathon Runners who had not yet finished the race are stopped after the explosions. Hide Caption 27 of 55 Photos: Deadly attack at Boston Marathon Victims are helped at the scene of the first explosion. Hide Caption 28 of 55 Photos: Deadly attack at Boston Marathon A man in tears is helped at the scene on Boylston Street. Hide Caption 29 of 55 Photos: Deadly attack at Boston Marathon Passersby put pressure on a victim's leg to try to stop the bleeding. Hide Caption 30 of 55 Photos: Deadly attack at Boston Marathon Massachusetts State Police guard an area near Kenmore Square. Hide Caption 31 of 55 Photos: Deadly attack at Boston Marathon Bystanders embrace near the finish line. Hide Caption 32 of 55 Photos: Deadly attack at Boston Marathon Police officers gather on Newbury Street. Hide Caption 33 of 55 Photos: Deadly attack at Boston Marathon Women and children are escorted away from the scene. Hide Caption 34 of 55 Photos: Deadly attack at Boston Marathon Bomb squad officials check a possible suspicious device near the scene of the blasts. Hide Caption 35 of 55 Photos: Deadly attack at Boston Marathon A woman is comforted after the blasts. Hide Caption 36 of 55 Photos: Deadly attack at Boston Marathon Two injured women are taken away on stretchers. Hide Caption 37 of 55 Photos: Deadly attack at Boston Marathon An injured woman is loaded into an ambulance. Hide Caption 38 of 55 Photos: Deadly attack at Boston Marathon First responders load injured people into an ambulance. Hide Caption 39 of 55 Photos: Deadly attack at Boston Marathon An injured man is prepared to be moved from a stretcher to an ambulance. Hide Caption 40 of 55 Photos: Deadly attack at Boston Marathon An injured woman is placed on a stretcher. Hide Caption 41 of 55 Photos: Deadly attack at Boston Marathon A runner is comforted following the attack. Hide Caption 42 of 55 Photos: Deadly attack at Boston Marathon The Cambridge Police Department's bomb squad investigates unattended personal items left behind after the explosions. Hide Caption 43 of 55 Photos: Deadly attack at Boston Marathon Bystanders check their mobile devices for news of the explosions. Hide Caption 44 of 55 Photos: Deadly attack at Boston Marathon A member of the bomb squad investigates a suspicious item on the road near Kenmore Square. Hide Caption 45 of 55 Photos: Deadly attack at Boston Marathon A runner in a wheelchair is taken from a triage tent after the explosions went off. Hide Caption 46 of 55 Photos: Deadly attack at Boston Marathon People comfort each near the site of the blasts. Hide Caption 47 of 55 Photos: Deadly attack at Boston Marathon Racers and race officials stand by after the explosions. Hide Caption 48 of 55 Photos: Deadly attack at Boston Marathon Emergency personnel respond to the scene. Hide Caption 49 of 55 Photos: Deadly attack at Boston Marathon Police and emergency crews tend to victims. Hide Caption 50 of 55 Photos: Deadly attack at Boston Marathon An injured woman is carried away on a stretcher. Hide Caption 51 of 55 Photos: Deadly attack at Boston Marathon A man lies on the ground after the incident. Hide Caption 52 of 55 Photos: Deadly attack at Boston Marathon Officials watch as the first explosion goes off on Boylston Street. Hide Caption 53 of 55 Photos: Deadly attack at Boston Marathon Spectators leave the bleachers after the explosions. Hide Caption 54 of 55 Photos: Deadly attack at Boston Marathon Police inspect one of the blast sites. Hide Caption 55 of 55

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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."

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"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.

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.