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 vowed that those responsible will "feel the full weight of justice."
As many as two unexploded bombs were also found near the end of the 26.2-mile course as part of what appeared to be a well-coordinated attack, but they were safely disarmed, according to a senior …show more content…
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. The person said the boy's mother and sister were also injured as they waited for his father to finish the race.
Hospitals reported at least 144 people injured, at least 17 of them critically. The victims' injuries included broken bones, shrapnel wounds and ruptured eardrums.
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. 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