(CNN) -- One of the deadly Boston Marathon bombs was apparently placed inside a metal pressure cooker and hidden in a backpack, a federal law enforcement official said Tuesday.

A timer was likely used to activate one of the bombs, rather than a cell phone, another law enforcement official said.

Details began to emerge as investigators combed through shreds of evidence in 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, 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 two bombs Monday turned a crowded celebration into devastation, killing three people including an 8-year-old boy. More than 170 people were wounded.

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, a federal law enforcement official in the intelligence community said.

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.

So far, no suspects or motive have been determined.

"We will go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime -- and we will do everything we can to bring them to justice," said Richard DesLauriers, the special agent in charge of the FBI's Boston Division.

"Our mission is clear: to bring to justice those responsible... The American public wants answers. The citizens of the city of Boston and the Commonwealth of Massachusetts want and deserve answers."

President Obama said that while it was "an act of terrorism," it's unknown whether the attack was the work of a terrorist group or "a malevolent individual."

Videos to be examined

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 bod," 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.

Authorities have not said what the bombs may have been made of.

Apartment searched

Authorities including bomb experts searched an apartment in nearby Revere, Massachusetts, and removed items overnight. But officials cautioned that the search did not suggest that there was a suspect.

The search was connected to a young Saudi citizen who is visiting on a student visa and has been questioned, a law enforcement official said, adding that consent was given and no warrant was needed. So far, the official told CNN, he has not heard of anything being found connecting the person to the bombings.

Three young Saudi men, all on student visas, live in the apartment, CNN affiliate WHDH reported. One of them, Mohammed Bada, told the station that when police arrived at the apartment, they told him that his roommate had been injured in the blasts.

Asked whether his friend was involved in the bombing, Bada told WHDH that he did not know, but that he does not think so. "They are good people," Bada said.

The experience of having police come to the apartment was "scary," he said.

The Revere Fire Department said on its Facebook page that the FBI; the Federal Bureau of Alcohol, Tobacco, Firearms, and Explosives; immigration officials, state and local police, detectives and bomb techs all took part in the search at the apartment, which lasted from early evening Monday until the early hours of Tuesday.

Investigators told police Monday to be on the lookout for a "darker-skinned or black male" with a possible foreign accent in connection with the marathon bombs, according to a law enforcement advisory obtained by CNN. The man was seen with a black backpack and sweatshirt and was trying to get into a restricted area about five minutes before the first explosion, the lookout notice states.

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 Police Commissioner Ed Davis said many people were being questioned.

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.

Nothing ruled out

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.

It isn't clear Monday whether the origin of the bombings was domestic or foreign.

Keating called the bombings a "sophisticated, coordinated, planned attack."

A law enforcement official in Boston said investigators "have a number of active leads and some good early progress in the forensics analysis."

There were no credible threats ahead of the race, a state government official said.

The FBI is taking the lead in investigating the attack near the marathon's finish line.

"This will be a combined federal, state and local effort," DesLauriers said.

Describing it a "criminal investigation" that is also "a potential terrorist investigation," DesLauriers said the FBI was declaring federal jurisdiction over the matter through the Boston Joint Terrorism Task Force.

Quick action helped preserve crime scene

Boston officials who worked quickly Monday to clear the crime scene and divert thousands of runners half a mile away should get an award, said Kayyem, who also served as homeland security adviser to Gov. Patrick.

The move minimized chaos and "preserved the crime scene, which is going to be key for the FBI investigation. Those are lessons learned out of 9/11."

Open events are hard to secure, Kayyem said. "People say, 'Oh, how could this happen again?...' The better way to look at it, I think, is: Did we respond better? I think the answer is yes."

"The situation remains fluid, and it remains too early to establish the cause and motivation," the FBI's Boston Division said in a statement asking people to call in with any information, images or details related to the explosions.

"No piece of information or detail is too small," it said.

CNN's Jethro Mullen and staff in Boston, New York John King, Matt Smith, Steve Almasy, and Monte Plott contributed to this report.