3 Killed as Bombs Explode at Boston Marathon, Obama Vows to Hold Culprits Accountable

Two bombs exploded in the crowded streets near the finish line of the Boston Marathon on Monday, killing at least three people and injuring more than 140 in a bloody scene of shattered glass and severed limbs that raised alarms that terrorists might have struck again in the U.S.

A White House official speaking on condition of anonymity because the investigation was still unfolding said the attack was being treated as an act of terrorism.

President Barack Obama, speaking from the White House late Monday, pointedly avoided using the words "terror" or "terrorism," saying officials "still do not know who did this or why." However, a White House official later said the explosions at one of the world's oldest and most prestigious race was being treated as terrorism.

"We will find out who did this. We'll find out why they did this," Obama said in his brief statement. "Any responsible individuals, any responsible groups, will feel the full weight of justice."

As the FBI took charge of the investigation, authorities shed no light on a motive or who may have carried out the bombings, and police said they had no suspects in custody. Officials in Washington said there was no immediate claim of responsibility.

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.

As many as two unexploded bombs were also found near the end of the 26.2-mile (42-kilometer) course as part of what appeared to be a well-coordinated attack, but they were safely disarmed, according to a senior U.S. intelligence official, who also spoke on condition of anonymity because of the continuing investigation.

The fiery twin blasts took place almost simultaneously and about 100 yards (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 course.

When the second bomb went off, the spectators' cheers turned to screams. As sirens blared, emergency workers and National Guardsmen assigned to the race for crowd control began climbing over and tearing down temporary fences to get to the blast site.

A pool of blood formed, and huge shards were missing from window panes as high as three stories.

"They just started bringing people in with no limbs," said runner Tim Davey, of Virginia. He said he and his wife, Lisa, tried to keep their children's eyes shielded from the gruesome scene inside a medical tent that had been set up to care for fatigued runners, but "they saw a lot."

Police said three people were killed. An 8-year-old boy was among the dead, according to a person who talked to a friend of the family and spoke on condition of anonymity.

Boston police said three people were killed. Hospitals reported at least 144 injured, at least 17 of them critically. The injuries ranged from cuts and bruises to amputations. Many victims suffered lower leg injuries and shrapnel wounds. Some suffered ruptured eardrums.

Some 23,000 runners took part in the race, which attracts more than 500,000 spectators and winds up in the heart of central Boston, near the landmark Prudential Center and the Boston Public Library. It is held on Patriots Day, a Massachusetts state holiday which commemorates the first battles of the American Revolution 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 precisely where the bombs were planted or whether they 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 president was briefed on the incident Monday by several senior administration officials, including FBI Director Robert Mueller and Homeland Security Secretary Janet Napolitano. He also spoke with Massachusetts Gov. Deval Patrick and Boston Mayor Tom Menino and pledged to provide whatever federal support was needed.

Additionally, the president spoke with Republican and Democratic leaders on Capitol Hill, saying that "on days like this, there are no Republicans or Democrats, we are Americans united in our concern for our fellow citizens."

The Federal Aviation Administration created a no-fly zone over the site of the explosions, and briefly ordered flights bound for Boston's Logan International Airport held on the ground at airports around the U.S.

A few miles (kilometers) away from the finish line and around the same time, a fire broke out at the John F. Kennedy presidential library. The police commissioner said it may have been caused by an incendiary device but didn't appear to be related to the bombings.

The first loud explosion occurred on the north side of Boylston Street, just before the photo bridge that marks the finish line. The second explosion could be heard a few seconds later.

They occurred about four hours into the race and two hours after the men's winner crossed the line. By that point, more than 17,000 of the runners had finished the race, but thousands of others were farther back along the course.

The four-hour mark is typically a highly crowded time near the finish line — both because of the slow-but-steady recreational runners likely to be completing the race and because of all the relatives and friends 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.

A senior U.S. intelligence official said the two other explosive devices found nearby were being dismantled. The official spoke on condition of anonymity because he was not authorized to discuss the findings publicly.

At the White House, the Secret Service expanded its security perimeter after the attacks, shutting down Pennsylvania Avenue and cordoning off the area with yellow police tape. Several Secret Service patrol cars blocked off entry points, although the White House was not on lockdown and tourists and other onlookers were still allowed in the park across the street.

At Congress, members of intelligence committees said they expected to be briefed on the attack on Tuesday.