Richard Rojas did not speak much about his three years in a Navy uniform, but when he returned to the Bronx from a naval base in Jacksonville, Fla., in 2014, he was a different man.
His mind was clouded with conspiracy theories. His dreams of opening his own clothing business had wilted. He lashed out at friends who challenged him; some thought his grasp of reality had slipped and that he needed psychiatric help.
During a string of arrests in recent years, Mr. Rojas once threatened to kill police officers, and last week he accused a notary of trying to steal his identity and grabbed his neck, the authorities said. But through it all, friends said, he never sought or received help, instead burrowing deeper into his paranoia and drinking or smoking marijuana.
On Thursday, Mr. Rojas, 26, was behind the wheel of a car that a friend said had been outfitted with a speeding detection system after a previous drunken-driving arrest. Under a wall of billboards and bright advertisements in Times Square, he made an abrupt U-turn before plowing through three and a half blocks of sidewalk crowds, killing an 18-year-old woman, Alyssa Elsman, and hurting 22 other people, the police said.