LATEST DEVELOPMENTS

— The death toll rises to three.

— More than 130 are injured, some critically.

— President Barack Obama vows justice for whomever is responsible.

— Security tightens across the country and around the world amid terrorism fears.

— Police say no one has claimed responsibility.

— The FBI takes charge of the investigation.

Two bombs exploded in the crowded streets near the finish line of the Boston Marathon on Monday, killing three people and injuring more than 130 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 vowed that those responsible will “feel the full weight of justice.”

A senior U.S. intelligence official said two other bombs were found near the end of the 26.2-mile course in what appeared to be a well-coordinated attack.

The fiery twin blasts took place about 10 seconds 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. Blood stained the pavement, and huge shards were missing from window panes as high as three stories above the course.

“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.”

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

Authorities shed no light on a motive or who may have carried out the bombings, and police said they had no suspects in custody. A person briefed on preliminary developments in the investigation said members of Boston’s Joint Terrorist Task Force were at Brigham and Women’s Hospital interviewing a wounded man seen running from the scene of the two blasts.

Authorities in Washington said there was no immediate claim of responsibility. The FBI took charge of the investigation.

Police said three people were killed. Hospitals reported at least 134 injured, at least 15 of them critically. The victims’ injuries included broken bones, shrapnel wounds and ruptured eardrums.

At Massachusetts General Hospital, said Alisdair 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. 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 cans.

He said authorities had received “no specific intelligence that anything was going to happen” at the race and declined to call the blasts a terrorist attack, though he said the department was treating them as such.

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 photo bridge that marks 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 to get to the blast site.

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 more than 4,000 others had passed Mile 24 and were on their way to the finish.

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.

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, 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.