BOSTON (Reuters) - 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.
"It sounded like a sonic boom. I haven't stopped shaking yet," said Melissa Stanley, who watched her daughter cross the finish line four minutes before the explosions.
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.
"I saw people who looked like they had their legs blown off. There was a lot of blood over their legs. Then people were being pushed in wheelchairs," said Joe Anderson, 33, a fisherman from Pembroke, Massachusetts, who had just run the race holding a large U.S. flag.
The blasts put police on alert in major cities across the United States, including in Washington, D.C. and New York City, sites of the September 11, 2001 hijacked plane attacks.
Four Boston area-hospitals contacted by Reuters reported a total of 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, who declined to be identified, said one or more bombs caused the explosions at the scene of the marathon, which is run annually on the state holiday Patriots' Day.
"These were powerful devices that resulted in serious injury," Boston Police Commissioner Ed Davis told reporters.
About an hour after the 2:50 p.m. EDT (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 three miles away, but no one was injured, police said. Authorities were uncertain whether the fire was related, Davis 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."
He said "any responsible individuals, any responsible groups will feel the full weight of justice."
No suspect was in custody. The U.S. Federal Bureau of Investigation, Justice Department, Homeland Security Department and other agencies were all investigating, authorities said.
Runners from the marathon and others went to the Massachusetts General Hospital offering to donate blood.
"This was evil, evil, evil," said Kevin Garboit, 46, from the hospital lobby, asking staff if he could donate blood. He was told to come back Tuesday morning.
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 yards (meters) 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 26.2 mile 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 50 feet in the air, Mitchell said. People began running and screaming after hearing the noise, Mitchell said.
"Everybody freaked out," Mitchell said.
The Boston Marathon has been held on Patriots' Day, the third Monday of April, since 1897. The event, which starts in Hopkinton, Massachusetts and ends in Boston's Copley Square, attracts an estimated half-million spectators and some 20,000 participants every year.
The Boston Symphony Orchestra cancelled Monday night's concert and the National Hockey League's Boston Bruins canceled their home game against the Ottawa Senators. The Boston Red Sox had completed their Major League Baseball game at Fenway Park before the explosions.
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.
Additional reporting by Tim McLaughlin, Aaron Pressman, Stephanie Simon, Edith Honan, Frank McGurty and Mark Hosenball; Writing by Daniel Trotta; Editing by Grant McCool