WASHINGTON — The surviving suspect in the Boston Marathon bombings told F.B.I. interrogators that he and his brother considered suicide attacks and striking on the Fourth of July as they plotted their deadly assault, according to two law enforcement officials.

But the suspect, Dzhokhar Tsarnaev, 19, told investigators that he and his brother,Tamerlan, 26, who was killed in a shootout with the police, ultimately decided to use pressure-cooker bombs and other homemade explosive devices, the officials said.

The brothers finished building the bombs in Tamerlan’s apartment in Cambridge, Mass., faster than they had anticipated, and so decided to accelerate their attack to the Boston Marathon on April 15, Patriots’ Day in Massachusetts, according to the account that Dzhokhar provided to authorities. They picked the finish line of the marathon after driving around the Boston area looking for alternative sites, according to this account.

On Friday morning, federal agents, state troopers and local law enforcement officers fanned out to search areas in the vicinity of Dartmouth, Mass., as part of their continuing investigation into the bombings, an F.B.I. spokesman, Jason J. Pack, said.