The blasts occurred in rapid succession as thousands of runners were nearing the finish line and sent scores 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 Patriots 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 prestigious race and is as much a symbol of Boston as Fenway Park.
At a news conference Monday night, Richard Deslauriers, special agent in charge for the FBI, described it as a "potential terrorist investigation." He declined to say how many other explosive devices were found. A senior U.S. intelligence official told The Associated Press that two other bombs were found near the end of the 26.2-mile course and were being dismantled.
As the FBI took charge of the investigation, authorities shed no light on a motive, and police said they had no suspects in custody. Officials in Washington said there was no immediate claim of responsibility.
WBZ-TV in Boston reported late yesterday that law enforcement officers were searching an apartment in the Boston suburb of Revere. Massachusetts State Police confirmed that a search warrant related to the investigation into the explosions was served last night in Revere but provided no further details.
An 8-year-old boy was among the dead, according to a person who talked to a friend of the family, and the boy's mother and sister were also injured as they waited for his father to finish the race.
Hospitals reported at least 144 people injured, at least 17 of them critically. The victims' injuries included broken bones, shrapnel wounds and ruptured eardrums. Many of the injured appeared to be spectators.
"Some were bleeding from the head; they had glass shards in their skin," said Nickilynn Estologa, a nursing student who was volunteering in a medical tent. "One person had the flesh gone from his leg; it was just hanging there."
In Washington, President Barack Obama vowed that those responsible will "feel the full weight of justice."
In New York City, NYPD officials and Mayor Michael Bloomberg ordered security ramped up at hotels, and transportation hubs such as Penn Station and Grand Central Terminal, as well as popular tourist destinations such as Times Square, Radio City Music Hall and the World Trade Center site.
Gov. Andrew M. Cuomo has directed state agencies to be on alert. And Suffolk and Nassau officials also took notice of the attacks, putting officers on heightened alert. Suffolk officials said they are paying special attention to the Long Island Rail Road.
Authorities shed no light on a motive or who may have carried out the bombings. Deslauriers said there is no suspect in custody despite media reports. CNN reported that a Saudi national with a leg wound was under guard at a Boston hospital, but investigators cannot say he is involved at this time.
The Federal Aviation Administration barred low-flying aircraft within 3.5 miles of the site.
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."
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 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. About 12 seconds later, a second explosion happened a half-block away. 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, of Indianapolis, who was about 75 yards from the finish line.
Matt Borowick, who lives in Crown Heights, Brooklyn, was in Boston to photograph and support a brother-in-law who was running in the race. "Nobody had cellphone service, and everyone was just shocked and confused and wanted to know what was going on," he said.
At Massachusetts General Hospital, chief of emergency services Alasdair Conn told The Associated Press: "This is something I've never seen in my 25 years here . . . this amount of carnage in the civilian population. This is what we expect from war."
Police Commissioner Edward Davis asked people to stay indoors or go back to their hotels and avoid crowds as bomb squads checked parcels and bags left along the race route. He said investigators didn't know whether the bombs were planted in mailboxes or trash cans.
Rep. Stephen Lynch (D-South Boston) said a security sweep was done in advance of the race, a routine check since 9/11. "They think it may have been devices brought in after the sweep was done," he said.
With Chau Lam, Tim Herrera and AP