BOSTON - Two bombs exploded at the venerable Boston Marathon on Monday, killing at least three people, injuring more than 140 others and rattling nerves around the nation, authorities said.

The blasts occurred in rapid succession as thousands of runners were nearing the finish line and sent scores of them scrambling for cover.

Video footage showed an explosion off to the side, with some runners toppling over from the concussion. Smoke billowed into the air, and photos of the chaotic scene showed a sidewalk slicked with blood.

The explosions, occurring on the Patriot's Day holiday that commemorates the Revolutionary War battles of Lexington and Concord, were as shocking in their symbolism as in their force. The Boston Marathon is a highly prestigious race and is as much a symbol of Boston as Fenway Park.

Two federal law enforcement officials, speaking on condition of anonymity, confirmed that the blasts were caused by explosive devices. One of the dead, according to one official, was an 8-year-old.

The repercussions extended from Massachusetts to Washington, where President Obama was briefed by top officials, the White House increased security, and the Justice Department and the FBI mobilized to fully investigate what had happened.

In a brief appearance at the White House shortly after 3 p.m. Tucson time, Obama expressed sympathy for the victims of the blasts and said all the necessary resources of the federal government would be assigned to assist Boston officials in determining the cause of the explosions.

"We still do not know who did this or why, and people shouldn't jump to conclusions before we have all the facts," Obama said. "But make no mistake - we will get to the bottom of this."

A White House official called the explosions an "act of terror," saying authorities have much to learn about who was behind it.

"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," the official said, speaking on condition of anonymity. "However, we don't yet know who carried out this attack, and a thorough investigation will have to determine whether it was planned and carried out by a terrorist group, foreign or domestic."

Initial reports of another explosion at the John F. Kennedy Library in Boston turned out to be an unrelated fire. Massachusetts Gov. Deval Patrick urged people to "stay out of crowds" as they made their way home.

The explosions ripped into an idyllic afternoon finish for the marathon. The first men had passed the finish line 2 hours and 10 minutes after the staggered start, and the first women crossed just 16 minutes later. But a bulge of slower runners grappling with a four-hour run time were converging on the race's end at 11:50 a.m. Tucson time.

The first blast sent a quick plume of smoke two stories high. Runners nearby stopped in their tracks, confused and unsure. After a few seconds later, a second explosion happened a half-block away, with a deep boom caught on television cameras.

Emergency personnel rushed to the area, and the street was quickly sealed off.

"I saw it go off and smoke billowed up. Everyone just stopped and hunched down," said Pam Ledtke, 51, from Indianapolis, who was about 75 yards from the finish line when the explosions went off. "They didn't know what to do," Ledtke said.

"All of a sudden, people were screaming," Ledtke added.

Nickilynn Estologa, a nursing student who was volunteering in a block-long medical tent designed to treat fatigued runners, said five to six victims immediately staggered inside. Several were children; one was in his 60s.

"Some were bleeding from the head; they had glass shards in their skin," she said. "One person had the flesh gone from his leg; it was just hanging there." Another woman, she added, was lying on a gurney as emergency personnel raced through the tent, giving her CPR.

"I just can't believe anyone would do something like this," Estologa said.

The explosions occurred shortly before noon Tucson time near the intersection of Boylston and Exeter streets. Local media reports said storefronts were blown out.

Many of the injured appeared to be spectators who were watching the race. About half of the nearly 27,000 participants had reportedly finished the race when the blasts occurred. The racers came from at least 56 countries and territories.

"I saw two explosions," reported Boston Herald journalist Chris Cassidy, who was running in the marathon. "The first one was beyond the finish line. I heard a loud bang and I saw smoke rising."

The blast "looked like it was in a trash can or something," he said. "There are at least a dozen that seem to be injured in some way."

Police established a crime scene around the Prudential Center, which is near the finish line. The blast apparently occurred about 300 yards from the finish line.

Authorities in New York and Washington tightened security precautions in the wake of the blasts. The Bureau of Alcohol, Tobacco, Firearms and Explosives sent all of its bomb technicians, explosives officers, explosives specialists and canine officers from their Boston and New York field divisions to the scene, as well as some investigators from Washington.

Shortly after being notified around noon, Obama received a briefing from homeland security adviser Lisa Monaco and other members of his senior White House staff in the Oval Office. The president called Boston Mayor Tom Menino and Massachusetts Gov. Patrick to express his concern for those who were injured and to make clear that his administration is ready to provide support.

Witnesses described a chaotic scene in the immediate aftermath of the blasts.

Paul Cummings, a 44-year-old runner from Portland, Ore., was in the medical tent near the finish line getting a leg massage when the explosions occurred.

"It didn't sound like a water main blowing or anything else - it sounded like a bomb," Cummings said. "Maybe I watch too much TV or something, but as soon as I heard it, I knew it was a bomb. It was just a loud explosion, and then another. You can't hear a noise like that and think anything good happened."

As police started bringing wounded people into the tent, Cummings quickly got up and left. "I just thought, 'I'm out of here.' "

He stepped out into Copley Square to wailing sirens, people shouting and crying and police imploring the crowds to leave the area.


• A 78-year-old runner is near the first blast but escapes with a scraped knee; the blasts occur when media exposure is maximized. Page A12

• Arizona lawmaker cites Boston as an example of citizens' need for firearms. Page A12

• Terror attack takes focus away from the marathon's winners. Page B1