A 19-year-old woman who was struck and killed Monday by a car on Alewife Brook Parkway was identified by State Police on Tuesday as Mary Barrios of Cambridge. Barrios was hit by a Toyota Corolla being driven by a 38-year-old Chelsea man around 7:40 p.m., authorities said. According to David Procopio, a State Police spokesman, Barrios might have been trying to cross the road when she was hit. The incident is under investigation and it is not yet clear whether the driver will face charges, Procopio said. Barrios was formerly of Chambersburg, Pa., where her family still lives, State Police said. She relocated to Cambridge but was not a student, Procopio said.