Blasts at Boston Marathon Kill 3 and Injure 100

Almost three-quarters of the 23,000 runners who participated in the race had already crossed the finish line when a bomb that had apparently been placed in a garbage can exploded around 2:50 p.m. in a haze of smoke amid a crowd of spectators on Boylston Street, just off Copley Square in the heart of the city. Thirteen seconds later, another bomb exploded several hundred feet away.

Pandemonium erupted as panicked runners and spectators scattered, and rescue workers rushed in to care for the dozens of maimed and injured, some of whom lost legs in the blast, witnesses said. The F.B.I. took the lead role in the investigation on Monday night, and Richard DesLauriers, the special agent in charge of the bureau’s Boston office, described the inquiry at a news conference as “a criminal investigation that is a potential terrorist investigation.”

The reverberations were felt far outside the city, with officials in New York and Washington stepping up security at important locations. Near the White House, the Secret Service cordoned off Pennsylvania Avenue out of what one official described as “an abundance of caution.”

President Obama, speaking at the White House, vowed to bring those responsible for the blasts to justice. “We will get to the bottom of this,” the president said. “We will find who did this, and we will find out why they did this. Any responsible individuals, any responsible groups will feel the full weight of justice.”

Mr. Obama did not refer to the attacks as an act of terrorism, and he cautioned people from “jumping to conclusions” based on incomplete information. But a White House official, speaking on the condition of anonymity afterward, said, “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.”

“However,” the official added, “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.”

Some runners were approaching the end of the 26.2-mile race when the two blasts, in rapid succession, sent them running away from the finish line.

“The first one went off, I thought it was a big celebratory thing, and I just kept going,” recalled Jarrett Sylvester, 26, a runner from East Boston, who said it had sounded like a cannon blast. “And then the second one went off, and I saw debris fly in the air. And I realized it was a bomb at that point. And I just took off and ran in the complete opposite direction.”

There were conflicting reports about how many devices there were. One law enforcement official said there had been four: the two that exploded at the marathon and two others that were disabled by the police. The official said that the devices appeared to have been made with black powder and ball bearings, but that investigators were unsure how the two that exploded had been set off.

It was unclear Monday evening who might be responsible for the blast. Although investigators said that they were speaking to a Saudi citizen who was injured in the blast, several law enforcement officials took pains to note that no one was in custody.

Some law enforcement officials noted that the blasts came at the start of a week that has sometimes been seen as significant for radical American antigovernment groups: it was the April 15 deadline for filing taxes, and Patriots’ Day in Massachusetts, the start of a week that has seen violence in the past. April 19 is the anniversary of the 1995 bombing of the Alfred P. Murrah Federal Building in Oklahoma City.