Two bombs ripped through the crowd at the finish line of the Boston Marathon on Monday, killing two people and injuring dozens in what a White House official said would be handled as an "act of terror."
President Barack Obama promised to hunt down whoever was responsible for the attack on a day when tens of thousands of spectators pack the streets to watch the world-famous race.
Many runners were heading for the finish when a fireball and smoke rose from behind cheering spectators and a row of flags representing the countries of participants, video from the scene showed.
The cheers turned to screams and panic.
Ambulances, fire trucks and dozens of police vehicles converged at the scene, and spectators could be seen crying and consoling each other.
The dead included an 8-year-old boy, the Globe reported, citing two law enforcement sources briefed on the investigation.
The blasts put police on alert in major cities including in Washington, D.C. and New York City, sites of the Sept. 11, 2001 hijacked plane attacks.
Four Boston area-hospitals reported at least 67 hurt. Some of those may have been hospitalized for treatment from running the marathon. The Boston Globe newspaper reported that more than 100 people were hurt.
Two high-level U.S. law enforcement officials said one or more bombs caused the explosions at the scene of the marathon, which is run annually on the state holiday Patriots' Day.
About an hour after the 2:50 p.m. (1850 GMT) blasts in Boston's Copley Square marred the usually joyous end to the marathon, a fire erupted at the John F. Kennedy Presidential Library 5 km away, but no one was injured, police said.
In Washington, Obama told reporters, "Make no mistake, we will get to the bottom of this and we will find out who did this."
No suspect was in custody. The FBI, Justice Department, Homeland Security Department and other agencies were all investigating, authorities said.
Without knowing who perpetrated the attack, the White House said it was handling the incident as "an act of terror."
"Any event with multiple explosive devices – as this appears to be – is clearly an act of terror, and will be approached as an act of terror," a White House official said.
The two explosions were about 50 to 100 metres apart as runners crossed the finish line with a timer showing 4 hours and 9 minutes, some 9 minutes faster than the average finish time, as reported by Runner's World magazine.
Of the 23,326 runners who started the race on Monday, 17,584 finished before the blast, marathon officials said. Runners were diverted before officials brought the marathon to a halt.
Spectators typically line the 42.2 km race course, with the heaviest crowds near the finish line.
Mike Mitchell of Vancouver, Canada, a runner who had finished the race, said he was looking back at the finish line and saw a "massive explosion."
Smoke rose 15 metres in the air, Mitchell said. People began running and screaming after hearing the noise, Mitchell said.
Earlier on Monday, Ethiopia's Lelisa Desisa and Kenya's Rita Jeptoo won the men's and women's events, continuing African runners' dominance in the sport.
