A Monday autopsy determined a 25-year-old man who was slain in Washington Heights died of multiple gunshot wounds, officials said. The still-unidentified victim was on a sidewalk in the 9500 block of South Genoa Avenue when someone shot him in his back and left leg about 4:25 a.m. Sunday, police said. He was pronounced dead about two hours later, at 6:29 a.m., at Advocate Christ Medical Center, according to the Cook County medical examiner's office. No arrests have been made; Area 2 detectives are investigating.