Police are searching for the gunman who shot and killed a 13-year-old in Brownsville overnight. Officials say Ronald Wallace was hit in the back at Tapscott and Black avenues, just blocks away from his home. He was rushed to Brookdale Hospital and pronounced dead. The teen's grandmother, Jackie Thornton, says she wonders if his murder may have had something to do with a run-in he had with another group of teens.The shooting marks the second time that a teenager has been gunned down in Brooklyn this week. No arrests have been made in either case. Anyone with information is asked to call Crime Stoppers at 1-800-577-TIPS.