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.

Two handguns and a BB gun that the authorities believe the brothers used in an earlier shootout with officers in Watertown were also recovered, said one official briefed on the investigation. The authorities said they believe the suspects had fired roughly 80 rounds in that shootout, in which Tamerlan Tsarnaev was fatally wounded, the official said.

Among the unanswered questions facing investigators are where the suspects acquired their weapons and explosives, how they got the money to pay for them, and whether others helped plan and carry out the attack last Monday. Mayor Thomas M. Menino of Boston said he believed the brothers were not affiliated with a larger network. “All of the information that I have, they acted alone, these two individuals, the brothers,” he said on ABC News’s “This Week.”

Some investigators said they believe the suspects used a design for the pressure-cooker bombs they allegedly detonated from a manual published in the online English-language magazine of Al Qaeda’s affiliate in Yemen. Mr. Menino said Tamerlan had “brainwashed” his younger brother to follow him and “read those magazines that were published on how to create bombs, how to disrupt the general public, and things like that.”

The suspects’ uncle Ruslan Tsarni, who lives in Maryland, said in an interview on Sunday that he had first noticed a change in the older brother in 2009. Mr. Tsarni sought advice from a family friend, who told him that Tamerlan’s radicalization had begun after he met a recent convert to Islam in the Boston area. Mr. Tsarni said he had later learned from a relative that his nephew had met the convert in 2007.

As scrutiny increased on how the brothers had been radicalized, Representative Michael McCaul, a Texas Republican who heads the Homeland Security Committee, and Representative Peter T. King, a New York Republican on the panel, sent a letter to the directors of three of the nation’s leading intelligence-gathering agencies calling the F.B.I.’s handling of the case “an intelligence failure.”