BOSTON -- A person briefed on twin explosions that tore through crowds at the finish line of the Boston Marathon Monday, said the explosives were in 6-liter pressure cookers and placed in black duffel bags.
The explosions killed at least three people and left 176 hurt, 17 of them critically.
The person said the explosives were placed on the ground and contained shards of metal, nails and ball bearings. The person, who spoke on condition of anonymity because the investigation was ongoing, said law enforcement officials have some of the bomb components, but did not yet know what was used to set off the explosives.
President Barack Obama said late Tuesday morning that the bombings were an "act of terror," but that investigators did not know if they were carried out by an international or domestic organization, or perhaps by a "malevolent individual."
Federal investigators said early Tuesday that no one had claimed responsibility for the bombings.
"We will find out who did this," President Obama said. "Any responsible individuals, any responsible groups, will feel the full weight of justice."
The Pakistani Taliban said Tuesday that it had nothing to do with the attack. Ehsanullah Esan, spokesman for the Taliban in Pakistan, told the AFP news agency that it was not involved in the attack. The Pakistani Taliban has claimed responsibility for the foiled 2010 Times Square bomb plot.
Immediately after the bombing, the FBI took charge of the investigation. Special Agent Rick Deslauriers said investigators were looking at possible terrorism, but he refused to comment on specific suspects or leads in the case.
Agents served a search warrant on an apartment in suburban Boston as part of their investigation Monday night. Massachusetts State Police said the warrant was served in Revere, but have not provided any details.
Investigators were seen leaving the apartment early Tuesday, carrying brown paper bags, plastic trash bags and a duffel bag.
The FBI is appealing for any video, audio or photos that may have been taken by marathon spectators in a search for any potential evidence that may have been in the background. Authorities are working to search surveillance camera footage from near Boston's Copley Square, scene of the bombings. So far, according to CNN, no footage has been spotted showing someone placing the bombs.
Conflicting reports emerged during the early hours of the investigation Monday about whether unexploded devices were found.
According to U.S. Rep. Bill Keating of Massachusetts, there were two such devices that did not detonate. One was found near the bomb site at a hotel on Boylston Street; the other was found at an undisclosed location, said Keating, a member of the House Homeland Security committee.
At a Tuesday morning news conference, FBI officials said there were no unexploded devices recovered.
The bombs 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.
The devices were described by the official as relatively small and possibly containing small ball bearings or BB gun pellets designed to serve as shrapnel. It was unclear whether the devices were remotely detonated or included timers, an official said, adding that no conclusions had been drawn on whether an organized group or lone wolf had been responsible for the attack.
The dead included an 8-year-old boy, The Boston Globe reported, citing law enforcement sources briefed on the investigation. Among the injured, 17 were reported in critical condition. The victims at eight local hospitals were as young as 2. According to CNN, at least 10 people had received amputations.
The stark pictures of mayhem and the injured sent over TV and the Internet also rekindled stark memories from the 9/11 terrorist attacks on New York's World Trade Center and the Pentagon. Monday's attacks also come just four days before the anniversary of the April 19, 1995 bomb attack on Oklahoma City's Murrah Federal Building.
The Boston Marathon blasts occurred about 12 seconds and 100 yards apart at about 2:50 p.m., three hours after Ethiopia's Lelisa Desisa won the 26.2-mile race.
Jim Davis, one of the marathon's official photographers, told The Des Moines Register he was about 50 feet from where the first blast ripped through a glass storefront.
"Debris was falling. Fortunately I was far enough away that there weren't any glass shards," said Davis, 65, of Fairfield, Iowa. "Then people started running and screaming and I realized this is not an accident - I should get out of there."
After the second explosion, about a block away, Davis returned for his camera gear and saw one man who had lost both his legs and others who were severely cut.
"I'm not a war correspondent," he said. "I'm not used to seeing people blown up with injuries."
Tom Beusse, president of the USA TODAY Sports Media Group, had just finished the race and was about 150 yards away from the explosion.
"There was this giant explosion. All of us turned around, the runners, and had these looks on their faces like 'Oh my God,'" he said. "Immediately, it turned into mayhem. People were screaming. Cops told us to keep moving away from the finish line in the direction we were going. No one knew what was coming next - and thankfully, nothing was next."
In other cities around the nation, heightened security was evident after the bombing in Boston. In Washington, Pennsylvania Avenue in front of the White House was cleared and lined with security tape.
Chief NYPD spokesman Paul Browne said critical response teams are deployed around the city until more about the explosion is learned. Officials are stepping up security at hotels and other prominent locations.
British police also say they are reviewing security plans for Sunday's London Marathon, the next major international marathon. A London Metropolitan Police spokesman says police are working with marathon officials to review security plans.
The National Hockey League announced late Monday afternoon that Monday night's game between the Ottawa Senators and the Boston Bruins, set to be played at Boston's TD Garden, had been postponed. The NBA later said that Tuesday's game between the Indiana Pacers and the Boston Celtics, also set for the TD Garden, would be canceled and not made up.