On the day after, the what and the how of the Boston Marathon bombings became clearer — explosive devices crafted from pressure cookers and stuffed with nails and ball bearings killed three people and injured 176 — but the who and the why remained a mystery.

With no one claiming responsibility for Monday’s attack, hundreds of investigators in Boston and Washington began combing through more than 2,000 video and still images of the race route, searching for clues that might help determine whether the bombings were an act of domestic or foreign terrorism, planned by an organized enemy or a lone actor.

“The range of suspects and motives remains wide open,” FBI Special Agent in Charge Richard Des­Lauriers said at an evening news conference in Boston. He said that the investigation is “in its infancy” and that evidence — including fragments of BBs and nails, as well as pieces of black nylon that could have been part of an “unusually heavy” backpack or bag holding the bombs — has been sent to the FBI lab at Quantico, where technicians will try to reconstruct the devices. He said it is not known how many people set off the bombs.

“It’s our intention to go through every frame of every video,” Boston Police Commissioner Ed Davis said.

In the first successful U.S. terrorist attack of the smartphone era, that means authorities face the daunting task of looking at thousands of images from phones, business- and government-owned surveillance cameras, and even runners’ head cameras.

Authorities urged that anyone with images of the area call 800-CALL-FBI (800-225-5324).

“We are particularly interested in reviewing video footage captured by bystanders with cellphones or personal cameras near either of the blasts,” said Attorney General Eric H. Holder Jr. “In an investigation of this nature, no detail is too small.”

The danger in any such investigation is that officials will be so overwhelmed by raw data that important clues may be missed. Philip Mudd, a former senior official at the FBI and the CIA, said that “99.4 percent of what you have is chaff. The wheat will emerge, but it could take a few days unless you get a break.”

Federal investigators are scrubbing every fragment from the bombs for clues about where its components were obtained and by whom.

President Obama, who plans to travel to Boston on Thursday for an interfaith service dedicated to the bomb victims, called the attack “heinous and cowardly” and termed it “an act of terrorism.”

Police said they do not have any suspects. After a briefing by intelligence officials, Sen. Saxby Chambliss (R-Ga.) said, “There are a lot of things that are surrounding this that build an indication that it may have been a domestic terrorist.” But neither he nor law enforcement officials cited any specific evidence pointing to a source of the attack.

Chambliss said a Saudi national who was injured in the bombings is considered a witness. The man, who is recuperating at a Boston hospital, is in his 20s and is in the United States on a Saudi scholarship to study in the Boston area. An official at the Saudi Embassy in Washington said the man cooperated with police and gave authorities permission to search his apartment.

Bostonians and the thousands of visitors in town for the marathon awoke Tuesday to learn that the bombs had hurt people in a particularly cruel way, leading to numerous leg amputations; that their fellow citizens had responded with immeasurable bravery and selflessness; and that what is often described as random violence had struck in very concentrated ways.

Two brothers injured by the blast each lost a leg, and not only was 8-year-old Martin Richard killed, but his mother and younger sister suffered devastating injuries.

In addition to Martin, Krystle Campbell, 29, a catering manager from Arlington, Mass., who had joined a friend to watch the marathon, died at the scene. Campbell was initially thought to have suffered only a leg injury, but she had given her photo identification to her friend to hold in her back pocket, and it is the friend who is recovering from the leg injury.

Officials have not released the name of a third fatality, but Boston University said late Tuesday that one of its graduate students was killed and that the school was awaiting permission from relatives before releasing the name.

The survivors are a collection of families and friends who were banded together first by the marathon and then by terror.

They formed, if fleetingly, a community, and they reacted as one: Runners still in shorts on a brisk day rushed to give blood — so many that hospitals stopped accepting donations. Bostonians used social media to invite stranded runners to spend the night in their homes. Some spectators ran not away from the explosions but toward them, grabbing onto any cloth they could find to stanch the flow of blood from people they had never seen before.

More of the injured would have died if strangers and emergency crews hadn’t immediately contained the bleeding, said Joseph Blansfield, program manager at the Boston Medical Center trauma unit. “Tourniquets are a difference maker; tourniquets can save a life,” he said.

As she supervised a triage center for runners still looking to recover belongings and make their way home, Boston Health Commissioner Barbara Ferrer said, “So many volunteers have stepped up in so many big ways and small ways with thousands of acts of kindness.”

Students at Boston College said they would walk the final five miles of the race route on Friday because, as organizers of the effort said, “we decide when our marathon ends.”

Away from the site of the bombings, Boston sprang back a bit on Tuesday with Red Sox Nation resiliency — at least as much as it could, given that 12 blocks of the city’s Back Bay neighborhood remained cordoned off as the most complex crime scene in city history.

Less than 24 hours after the attack, diners sat at tables adorned with white napkins outside 5 Napkin Burger. At Dartmouth and Stuart streets, the corner Starbucks was offering free coffee and pastries. A few blocks away, city officials delivered 1,500 gear bags that had been waiting at the finish line for runners who didn’t get to complete the race.

Along several city blocks, investigators continued the search for physical and digital clues. A federal law enforcement official said the FBI was examining cellphone activity on towers near the scene — data that could be useful if the explosives were detonated by phone.

If the data yield suspicious numbers, investigators can get the subscriber’s billing information. If the phone was a disposable model, it’s possible to learn where it was bought. Surveillance cameras near the point of sale could provide further clues, former prosecutors said.

Rep. Michael McCaul (R-Tex.), chairman of the House Homeland Security Committee, said the bombs were akin to the improvised explosive devices deployed against U.S. forces in Iraq and Afghanistan and were “most likely dropped in a backpack.”

Police had swept the marathon route for explosives twice on Monday before the race, said Michael Bouchard, a former assistant director of the Bureau of Alcohol, Tobacco, Firearms and Explosives who is president of Security Dynamics Group.

But “anyone carrying a backpack or duffel bag could have carried the bombs in,” he said. “I’m sure there were hundreds of bags left on the ground during the race. In a road race, everyone has to have a change of clothes, another pair of shoes. . . . Because of the nature of the event, you’re allowed to have backpacks. That probably will change, unfortunately.”

Near the bomb site, some of the marathon’s 27,000 runners, many still wearing their blue-and-yellow Boston Marathon jackets, filled outdoor cafes, their celebration muted by loss and hurt.

“Today’s usually about still celebrating, just like last night would have been,” said Tim Walline, 48, an eye surgeon from Kansas City, Mo. “But we were in our hotel, kind of shut down.” He has decided about next year: “I’ll back, of course. Don’t let the bad guys get the upper hand.”

Fisher reported from Washington. David A. Fahrenthold, David Montgomery and Mary Beth Sheridan in Boston and Julie Tate, Alice Crites, Amy Argetsinger, Peter Finn, Sari Horwitz, Carol D. Leonnig, Greg Miller and Ellen Nakashima in Washington contributed to this report.