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.
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 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.