The children's father, Bill, is the director of a local community group. The boy's mother, Denise, works at the Neighborhood House Charter School, where her children attend classes.

At the White House, President Barack Obama vowed Monday that those responsible will "feel the full weight of justice."

A senior U.S. intelligence official had said two other explosive devices were found near the end of the 26.2-mile course, but on Tuesday officials said that the only explosive devices were the two that exploded.

The injuries ranged from cuts and bruises to amputations. Many victims suffered lower leg injuries and shrapnel wounds. Some suffered ruptured eardrums.

A doctor at a hospital where victims were taken said an X-ray of one victim he saw showed what could be small ball bearings throughout the injury.

Dr. Stephen Epstein of the Department of Emergency Medicine at Beth Israel Deaconess Medical Center said in an email to The Associated Press that the X-ray of a leg he saw has "what appears to be small, uniform round objects throughout it -- similar in the appearance to BBs."

He says exactly what the objects were remains to be determined.

Dr. Richard Wolfe, the emergency department chief at Beth Israel Deaconess, said Monday that one or two of the hospital's 21 patients faced a "high probability of mortality."

Runner Tim Davey, of Richmond, Va. 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.

"They just started bringing people in with no limbs," he said, adding that his children "saw a lot."

"They just kept filling up with more and more casualties," Lisa Davey said. "Most everybody was conscious. They were very dazed."

There was no word on the motive or who may have launched the attack. Police said no suspect was in custody, although later Monday night a person of interest was being questioned, according to multiple reports. Authorities in Washington said there was no immediate claim of responsibility.

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.

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.

The twin blasts at the race took place almost simultaneously and about 100 yards apart, tearing limbs off numerous people, knocking spectators and at least one runner off their feet, shattering windows and sending smoke rising over the street.

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."

Some 23,000 runners took part in the race, one of the world's oldest and most prestigious marathons. One of Boston's biggest annual events, the race winds up near Copley Square, not far from the landmark Prudential Center and the Boston Public Library.

The Boston Athletic Association extended its "deepest sympathies to all those who were affected in any way by today's events."

"... We can confirm that all of the remaining runners who were out on the course when the tragic events unfolded have been returned to a community meeting area," the association said in a release. "At this time, we are cooperating with the City of Boston, the Commonwealth of Massachusetts, and all federal law enforcement officials. We would like to thank the countless people from around the world who have reached out to support us today."

Boston police commissioner Ed 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 planted 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 from within 3.5 miles of the site.

Obama was briefed on the explosions by Homeland Security adviser Lisa Monaco. Obama also told Mayor Tom Menino and Gov. Deval Patrick that his administration would provide whatever support was needed, the White House said.

"We still don't know who did this or why," Obama said, adding, "Make no mistake: We will get to the bottom of this."

Also Monday, a fire broke out at the John F. Kennedy Library about five miles from the race finish line. The police commissioner said it may have been caused by an incendiary device but it didn't appear to be related to the bombings. Earlier, officials had said that a third explosive device had gone off at the library and appeared linked to the race scene. It was one of many erroneous reports that emerged from the chaotic scene.

"Fire in building is out, appears to have started in the mechanical room of new building. All staff and visitors are accounted for and safe," a tweet on the library's Twitter account read.

Davis said that the bomb squad was examining parcels left along the race route, many of which likely came from spectators watching the race.

"At this point, we have not found another device on Boylston Street," Davis said.

Shalane Flanagan, who finished fourth in the women's elite division, called the explosions "devastating."

"It's supposed to be something that unites people and brings them together," the American said. "It's a celebration of heroes, a celebration of people's achievements and dedication. It's overall an unbelievable, positive event that inspires so many people. This will surely taint it in a really sad way. It's pretty devastating that this happened."

About four hours into the race and two hours after the men's winner crossed the line, there was a loud explosion on the north side of Boylston Street, just before the photo bridge that marks the finish line. Another explosion could be heard a few seconds later.

The finish of the Boston Marathon became a chaotic scene after two explosions detonated in quick succession Monday afternoon. AP Photo/Charles Krupa

By that point, more than 17,000 of the runners had finished the race, but thousands of others were farther back along the course.

"There are people who are really, really bloody," said Laura McLean, a runner from Toronto who was in the medical tent being treated for dehydration when she was pulled out to make room for victims.

Another explosion was heard about an hour after the first two after authorities warned spectators to expect a loud noise from a water cannon as police performed a "controlled explosion," Davis said.

Vice President Joe Biden was on a conference call with gun control activists when staffers turned on televisions in his office Monday to view coverage of the explosions. Biden said during the call that his prayers were with those who suffered injuries.

The NHL postponed Monday night's game between the Ottawa Senators and Boston Bruins at TD Garden in the aftermath. The league said it "wishes to express its sympathy to all affected by the tragic events that took place in Boston earlier this afternoon."

The Boston Red Sox, who played a home game earlier Monday afternoon, seemed unaware of the explosions as they were interviewed by reporters after the game. In the Red Sox room, they dressed in suits and ties for their trip to Cleveland, where they're scheduled to start a three-game series against the Indians on Tuesday night. A team spokesman sent a text message saying the team had reached the airport.

The Celtics had a scheduled off day Monday but canceled Tuesday night's home game against Indiana.

Bill Iffrig, 78, was knocked to the ground by the initial blast, a stumble that was caught on tape in a video of the explosion.