AP Source: Boy, 8, among dead in Boston Marathon blasts

Two bombs exploded in the packed streets near the finish line of the Boston Marathon on Monday, killing at least three people in a terrifying scene of shattered glass, bloodstained pavement and severed limbs, authorities said.

A person familiar with the situation tells The Associated Press that an 8-year-old-boy is among the dead.

The person said the boy's mother and sister were also injured as they waited for his father to finish the race.

Police commissioner Ed Davis confirmed the three deaths but provided no details about the ages of the victims.

The explosions injured at least 144 people, 17 critically.

Some of the victims lost arms and legs. Other injuries included broken bones, shrapnel wounds and ruptured eardrums.

A senior U.S. intelligence official said two other bombs were found near the end of the 26.2-mile course.

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

There was no word on the motive or who may have carried out the attack, and police said they had no suspects in custody. Authorities in Washington said there was no immediate claim of responsibility.

David Schanzer, director of the Triangle Center on Terrorism and Homeland Security, said an event like the Boston Marathon would be difficult to secure completely.

Although there is a crowded crush at the finish line, runners, volunteers and spectators are spread across the 26.3 miles of the course.

The twin blasts at the race took place almost simultaneously and about 100 yards apart about 4 hours and 10 minutes after the race had started. Marathon officials said the average finish, therefore the maximum number of people finishing the race, would happen at about 4 hours and 18 minutes.

The blasts tore limbs off numerous people, knocked spectators and runners off their feet, shattered windows and sent smoke rising over the street.

"They just started bringing people in with no limbs," said runner Tim Davey, of Richmond, Va. 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."

A third explosion was heard about an hour after the first two after authorities warned spectators to expect a loud noise from a water cannon that police apparently were using to destroy one of the devices. 

Competitors and race volunteers were crying as they fled the chaos. Authorities went onto the course to carry away the injured while race stragglers were rerouted away from the smoking site.

Some 27,000 runners took part in the 26.2-mile race, one of the world's premier marathons. Many more cheering fans line the course annually to mark Patriot's Day, a local Boston holiday. Just across the street from the blast, fans were exiting Fenway Park after a game between the Boston Red Sox and Tampa Bay Rays.

"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 of the explosions. "They were pulling them into the medical tent."

Runners who had not finished the race were diverted straight down Commonwealth Avenue and into a family meeting area, according to an emergency plan that had been in place.

The Federal Aviation Administration barred low-flying aircraft from within 3.5 miles of the site.

President Barack 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.

The White House was not on lockdown and tourists and other onlookers were still able to be in the park across the street from the executive mansion.