A 16-year-old Milwaukee boy was gunned down while singing outside his south side home, police said Monday. Tony Mercado died outside his home in the 1900 block of S. 28th St., police said. No one was arrested in connection with the shooting Sunday, but police say they believe the youth knew his killers. About 9:30 p.m., Mercado and a relative were sitting on the curb outside their home when a group of males on bicycles approached him, the relative said. Mercado was singing in the street, something he did frequently, she said. The group tried to pick a fight with...