A 27-year-old man was fatally stabbed outside his Spruce Street home Sunday night, and the assailant remains at large, authorities said. Paul Frontiero was attacked around 10:30 p.m. Sunday, police said. Two women, Jill Arnold, 29, and Kathryn Libby, 29, were also injured. Libby suffered serious stab wounds but is expected to survive. Arnold suffered minor cuts. Frontiero was taken to Southern New Hampshire Medical Center, where he was pronounced dead. An autopsy was to be conducted yesterday. His death has been declared a homicide. Investigators say a man fled the scene of the attack in a car with at least one other person.