BOSTON -- Three people died and more than 140 were injured in two powerful blasts at the Boston Marathon finish on Monday afternoon that the FBI said have become a "potential terrorist investigation."
The Massachusetts governor, Boston police commissioner, and FBI's Boston bureau agent did not reveal further details at a Monday evening news conference of possible suspects, causes of the explosions, or evidence they've recovered.
No information on the identities of victims was released. Officials would not comment on media reports that an 8-year-old boy had been killed in the blasts.
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 FBI is taking the lead in the investigation, and special agent in charge Richard DesLauiels said the bureau is bringing in "substantial federal" resources. Authorities are asking anyone with information to call the tip line 1-800-CALL-FBI.
Authorities in Washington said there was no immediate claim of responsibility. Investigators were interviewing numerous witnesses; they asked for the public's help in turning over video or photos of the scene.
Governor Patrick said blocks around Boylston were blocked off and being treated as a crime scene. All abandoned property in the area -- bags cast off by runners and onlookers who fled the scene -- was being checked by law enforcement. The National Guard is limiting access to the area, he said.
"The city of Boston is open, but it will not be business as usual," he said. There's a heightened law enforcement presence. Belongings of riders on the MBTA may be subject to random searches, he said.
The governor praised the stories he'd heard of kindness that residents and business people had extended to the runners who found their race abruptly ended and were stranded. The city had set up a center where runners were being bused to be reunited with their loved ones.
Police Commissioner Edward Davis said it was too early in the investigation to answer many questions. But, he said, "This cowardly act will not be taken in stride."
The news conference was held at the Westin Copley hotel, near the scene. In the lobby and the second floor, dazed runners milled around, some sitting with luggage. The usual euphoric atmosphere at the end of any marathon was absent. Runners were subdued, their faces stricken.
Outside, SWAT vehicles, National Guard, and federal, state and local law enforcement filled the streets, their presence a jarring view of a usually bustling and lively area in the city.
The next official update on the bombings is scheduled for Tuesday at 9:30 a.m.
With Associated Press reports