The chief FBI agent in Boston vowed "we will go to the ends of the Earth" to find whoever carried out the deadly attack on one of the city's most famous civic holidays, Patriots Day. (Read)
A person briefed on the attack, which left the streets splattered with blood and glass, said the explosives were in 6-litre pressure cookers and placed in black duffel bags that were placed on the ground. The person said the duffel bags contained shards of metal, nails and ball bearings. The person spoke on the condition of anonymity because the investigation was ongoing.
Richard DesLauriers, FBI agent in charge in Boston, said earlier that investigators had received "voluminous tips" and were interviewing witnesses and were analyzing the crime scene.
"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," he said.
A European security official said Tuesday initial evidence indicates that the attacks were not the work of suicide bombers.
"So far, investigators believe it was not the work of suicide bombers, but it is still too early to rule it out completely," said the official, who spoke from the United States on condition of anonymity because he was not authorized to speak about the U.S. investigation.
The Pakistani Taliban, which has threatened attacks in the United States because of its support for the Pakistani government, on Tuesday denied any role in the marathon bombings.
The fiery explosions took place about 10 seconds and about 100 yards (90 meters) apart, knocking spectators and at least one runner off their feet, shattering windows and sending dense plumes of smoke rising over the street and through the fluttering national flags lining the route. (Pics)
Blood stained the pavement, and huge shards were missing from window panes as high as three stories.
Victims suffered broken bones, shrapnel wounds and ruptured eardrums.
Defense Secretary Chuck Hagel called the bombing a "cruel act of terror." He said that any event with explosive devices is clearly an act of terror and he promised that a thorough investigation will determine whether the perpetrators were foreign or domestic. The Pentagon chief vowed that those responsible will be brought to justice.
Massachusetts Gov. Deval Patrick said Tuesday no unexploded bombs were found at the marathon site. He said the only explosives were the ones that went off Monday.
Roupen Bastajian, a state trooper from Smithfield, Rhode Island, had just finished the race when he heard the explosions.
"I started running toward the blast. And there were people all over the floor," he said. "We started grabbing tourniquets and started tying legs. A lot of people amputated."
At Massachusetts General Hospital, Alasdair Conn, chief of emergency services, said: "This is something I've never seen in my 25 years here ... this amount of carnage in the civilian population. This is what we expect from war."
WBZ-TV reported late Monday that law enforcement officers were searching an apartment in the Boston suburb of Revere. Massachusetts State Police confirmed that a search warrant related to the investigation into the explosions was served Monday night in Revere, but provided no further details.
Some investigators were seen leaving the Revere house early Tuesday carrying brown paper bags, plastic trash bags and a duffel bag.
Investigators refused to give any specifics on the bombs and say, for example, where they might have been hidden or whether they were packed with shrapnel for maximum carnage, as is often the case in terror bombings overseas.
But Dr. Stephen Epstein of the emergency medicine department at Beth Israel Deaconess Medical Center, said he saw an X-ray of one victim's leg that had "what appears to be small, uniform, round objects throughout it - similar in the appearance to BBs." He said it remained to be determined what exactly the objects were.
Police said three people were killed. Eight-year-old Martin Richard was among the dead, according to a person who talked to a friend of the family and spoke on condition of anonymity. The person said the boy's mother and sister were also injured.
Police commissioner Ed Davis said 176 victims were brought to hospitals around Boston, and 17 were in critical condition. At least eight children were being treated at hospitals.
Tim Davey of Richmond, Virginia, was with his wife, Lisa, and children near a medical tent that had been set up to care for fatigued runners when the injured began arriving. "They just started bringing people in with no limbs," he said.
"Most everybody was conscious," Lisa Davey said. "They were very dazed."
The Boston Marathon is one of the world's oldest and most prestigious races and about 23,000 runners participated. The race honored the victims of the Newtown, Connecticut, shooting with a special mile marker in Monday's race.
Boston Athletic Association president Joanne Flaminio previously said there was "special significance" to the fact that the race is 26.2 miles long and 26 people died at Sandy Hook Elementary School.
One of the city's biggest annual events, the race winds up near Copley Square, not far from the landmark Prudential Center and the Boston Public Library. It is held on Patriots Day, which commemorates the first battles of the American Revolution, at Concord and Lexington in 1775.
Boston Police Commissioner Edward Davis asked people to stay indoors or go back to their hotel rooms and avoid crowds as bomb squads methodically checked parcels and bags left along the race route. He said investigators didn't know whether the bombs were hidden in mailboxes or trash cans.
He said authorities had received "no specific intelligence that anything was going to happen" at the race.
The Federal Aviation Administration barred low-flying aircraft within 3.5 miles (5.6 kilometers) of the site.
"We still don't know who did this or why," Obama said at the White House, adding, "Make no mistake: We will get to the bottom of this."
With scant official information to guide them, members of Congress said there was little or no doubt it was an act of terrorism.
"We just don't know whether it's foreign or domestic," said Republican Rep. Michael McCaul, chairman of the House Committee on Homeland Security.
A few miles away from the finish line and around the same time, a fire broke out at the John F. Kennedy Library. The police commissioner said that it may have been caused by an incendiary device but that it was not clear whether it was related to the bombings.
The first explosion occurred on the north side of Boylston Street, just before the finish line, and some people initially thought it was a celebratory cannon blast.
When the second bomb went off, spectators' cheers turned to screams. As sirens blared, emergency workers and National Guardsmen who had been assigned to the race for crowd control began climbing over and tearing down temporary fences to get to the blast site.
The bombings occurred about four hours into the race and two hours after the men's winner crossed the finish line. By that point, more than 17,000 of the athletes had finished the marathon, but thousands more were still running.
The attack may have been timed for maximum carnage: The four-hour mark is typically a crowded time near the finish line because of the slow-but-steady recreational runners completing the race and because of all the friends and relatives clustered around to cheer them on.
Runners in the medical tent for treatment of dehydration or other race-related ills were pushed out to make room for victims of the bombing.