Law enforcement officials, who asked the public to turn over any photos or video of Monday's marathon and the blasts, did not disclose any possible leads in the investigation. No one has been arrested, police said.
Late on Monday, police searched a Boston area apartment of a Saudi Arabian student who was injured in the blast, law enforcement sources said. But they said evidence showed that the student was expected to be cleared of suspicion and that he was unlikely to shed any light on the attack.
Numerous other theories and leads in the investigation are being looked at, according to the same sources, who asked not to be identified.
Current and former counter-terrorism officials said that the Boston bombs were built using pressure cookers as the superstructure, black powder or gunpowder as the explosive and ball bearings as additional shrapnel. The officials said that instructions on how to design such bombs are available on the Internet.
Obama, in an appearance in the White House briefing room, said it was not yet clear who carried out Monday's twin blasts that killed three people and sent 176 to hospitals with injuries, 17 critical.
"Any time bombs are used to target innocent civilians, it is an act of terror," Obama said. "What we don't yet know, however, is who carried out this attack or why, whether it was planned and executed by a terrorist organization - foreign or domestic -or was the act of a malevolent individual."
NO OTHER DEVICES FOUND
Dispelling earlier reports of as many as seven devices being found around Boston, Gene Marquez, assistant special agent in charge for the US Bureau of Alcohol, Tobacco and Firearms, said authorities had determined that the only bombs deployed in the attack were the two that detonated shortly before 3 pm EDT (1900 GMT) on Monday.
Any unexploded device might have provided a clearer picture of what materials were used and how the bomb was assembled, furnishing leads in the case.
Meanwhile, a stretch of Boylston Street near the race's finish line, where the blasts occurred, and the blocks around it were closed to traffic as police searched for evidence.
A banner that had marked the finish line still hung over the deserted street.
Responding to the worst attack on US soil since security was tightened after the September 11, 2001 hijacked plane strikes, Obama urged Americans to be vigilant.
The United States was clearly on heightened alert on Tuesday.
At Boston Logan International Airport, two passengers and their bags were removed from a United Airlines flight before departure on Tuesday morning, a source with direct knowledge of the action said.
Meanwhile, a US Airways plane arriving at Logan from Philadelphia was parked in a corner of the airport while a suspicious bag was inspected, airport officials said. The bag was later found to be harmless.
In New York, bomb squad investigators were called in and the central terminal of La Guardia International Airport was evacuated after authorities discovered a suspicious package at around 10:30 a.m. EDT (1430 GMT) in the terminal, authorities said.
Trauma surgeons at several Boston hospitals said at press briefings the majority of victims suffered lower-body injuries, and several had a range of metallic shrapnel material removed during surgery, including pellets and what appeared to be carpenter nails.
"The vast majority of the injuries were to lower extremities, including some victims who had parts of their legs blown off," said Dr. Tracey Dechert, a trauma surgeon at Boston Medical Center, which treated 23 people and performed amputations on five of them.
The inclusion of material such as nails in the device would be reminiscent of the 1996 bombing at the Summer Olympics in Atlanta, which killed two people and injured about 150 others. Anti-abortion activist Eric Rudolph, who eluded capture for years, pleaded guilty to the attack and is currently serving consecutive life sentences.
An 8-year-old boy was among the dead. He was identified as Martin Richard in a statement issued by his father, Bill Richard.
Outside the family home in the Dorchester neighborhood of Boston, people left flowers on the doorstep and someone had written "peace" in chalk on the sidewalk.
"All of Dorchester is devastated. The whole world is devastated," said Jane Sherman, who lives next door. "I don't know if there are any words I can say. I don't know if there's anything anyone can say."
The names of the other two people killed have not been released by authorities.
A 2-year-old was being treated at Boston Children's Hospital for a head wound, the hospital said.
Officials in Britain and Spain said the London and Madrid marathons would go ahead on Sunday, but security plans for both races were under review.
Runners who had traveled to Boston for the annual marathon, which has been held since 1897 and attracts an estimated half-million spectators and some 20,000 participants, remained in shock on Tuesday morning.
Pat Monroe-DuPrey, of Winter Haven, Florida, ran with his wife, Laura, in a trip to mark their 10th anniversary after being married during the race.
He said he did not know what to make of the blast, which came as he was finishing the race in a state of exhaustion.
"You don't have a brain at 26 miles," Monroe-DuPrey said. "They got us off the course, and then I was panicking."