The death of a 22-year-old man who was shot early Sunday in the South End is being investigated, Boston police said Tuesday. Around 3:40 a.m. Sunday, officers received a call for a shooting in the area of 1050 Tremont St. When they arrived, they found the man with a gunshot wound, police said. The unidentified victim was taken to Boston Medical Center, where he was pronounced dead, police said. Police said a firearm was recovered at the scene. No arrests have been made.