BOSTON — Two bombs exploded at the venerable Boston Marathon on Monday, killing two people, injuring about 100 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 FBI mobilized to fully investigate what had happened.

In a brief appearance at the White House shortly after 6 p.m., 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 ofslower runners grappling with a four-hour run time were converging on the race’s end at 2:50 p.m.
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 just can’t believe anyone would do something like this,’’ Estologa said.
The explosions occurred shortly before 3 p.m. near the intersection of Boylston and Exeter streets. Local media reports said store fronts 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 3 p.m., 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. Deval 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, Oregon, 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.”