WASHINGTON — The surviving suspect in the Boston Marathon bombings told F.B.I. interrogators that, as he and his brother plotted their deadly assault, they considered suicide attacks and striking on the Fourth of July, 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 anticipated, and so decided to accelerate their attack to...