A tractor-trailer driver killed a 19-year-old moped rider in Brooklyn on Saturday morning, then fled the scene before turning himself in to the police later, authorities said. The victim, Josue Mardoqueo Vicente Yac, was riding south on Fourth Avenue around 2:30 a.m. when a 2002 Peterbilt semitruck made a left turn from the same direction onto 39th Street and struck him. Vicente Yac suffered severe head and body injuries and died at NYU Langone Hospital—Brooklyn. The 41-year-old truck driver, whose name was not released, walked into an NYPD stationhouse hours after the crash and told the police he might have been involved.