美国联邦调查局对波士顿马拉松致命的爆炸案展开调查

U.S. federal investigators are working to uncover who was behind a twin bombing Monday at the Boston Marathon that left at least three people dead and more than 140 others injured.

The Federal Bureau of Investigation is leading the multi-agency effort to investigate what a White House official said was "clearly an act of terror."

The FBI said in a statement late Monday it was too early to establish the cause or motivation for the explosions, which happened moments apart within about 100 meters of each other near the finish line.

VOA correspondent Carolyn Presutti said early Tuesday investigators were working in a wide area around the scene, with barricades on street corners to keep out people and traffic. 

She described a lot of security around the city with the sights and sounds of police cars and sirens, but otherwise no one out on the streets. 

U.S. President Barack Obama said in televised remarks that the United States will find out who was responsible for the blasts and hold them accountable.

Boston Police Commissioner Edward Davis said there was "no specific intelligence" warning of an attack, and authorities said there was no immediate claim of responsibility.

The blasts took place about four hours into the race, long after the winners had finished, but at the time that the highest number of runners and their supporters are usually around the finish line area. The competition, which had more than 23,000 runners, was halted after the bombs went off.

Bloody spectators, including some with severed limbs, were carried to the medical tent that had been set up to care for fatigued runners from the race.

The Boston Globe, citing two law enforcement sources, reported that the dead included an eight-year-old boy.  A number of victims suffered amputations.

Cities worldwide stepped up security following the explosions.