More details of what the authorities said was the original plot were becoming clearer. The Boston police commissioner, Edward Davis, said the authorities believed that Mr. Tsarnaev and his older brother, Tamerlan, 26, had planned more attacks beyond the bombings at the finish line of the Boston Marathon, which killed three people and wounded more than 170. When the suspects seized a Mercedes-Benz sport utility vehicle and held the driver hostage, they told him that they planned to head to New York, the senior United States official said Sunday.

It was not clear whether the suspects had told the driver what they planned to do there.

Mr. Davis told CBS News’s “Face the Nation” on Sunday: “We have reason to believe, based upon the evidence that was found at that scene — the explosions, the explosive ordnance that was unexploded and the firepower that they had — that they were going to attack other individuals.”

Along with determining that the suspects had made at least five pipe bombs, the authorities recovered four firearms that they believe the suspects used, according to a law enforcement official. The authorities found an M-4 carbine rifle — a weapon similar to ones used by American forces in Afghanistan — on the boat where the younger suspect was found Friday night in Watertown, Mass., 10 miles west of Boston.