About 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 authorities had received “no specific intelligence that anything was going to happen” at the race.
“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 Rep. Michael McCaul, R-Texas, 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 it may have been caused by an incendiary device but didn't appear to be related to the bombings.
The first explosion occurred on the north side of Boylston Street, just before the finish line.
When the second bomb went off, the 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.
The bombings 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 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 relatives and friends clustered around to cheer them on.
The senior U.S. intelligence official said the two other explosive devices found nearby were being dismantled. The official spoke on condition of anonymity.
AT A GLANCE
Setting the scene
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 woman who was a few feet from the second bomb, Brighid Wall, 35, of Duxbury, said that when it exploded, runners and spectators froze, unsure of what to do. Her husband threw their children to the ground, lay on top of them and another man lay on top of them and said, “Don't get up, don't get up.”
After a minute or so without another explosion, Wall said, she and her family headed to a Starbucks and out the back door through an alley. Around them, the windows of the bars and restaurants were blown out. She said she saw six to eight people bleeding profusely, including one man who was kneeling, dazed, with blood trickling down his head. Another person was on the ground covered in blood and not moving.
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.
Roupen Bastajian, a state trooper from Smithfield, R.I., had just finished the race when he heard the blasts.
“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 least 25 to 30 people have at least one leg missing, or an ankle missing, or two legs missing.”