BROOKLYN - Police are investigating a possible early morning sexual assault in Sunset Park.According to investigators, a 23-year-old woman was waiting for her friends at 53rd Street and 5th Avenue at around 4:30 a.m. when a man armed with a knife approached her and threatened to kill her. At first, the woman was able to get away from her attacker, but the man followed her and possibly sexually assaulted her, police say. The 23-year-old was treated at Coney Island Hospital.So far, no arrests have been made.