KAUFMAN, Texas (Reuters) - Authorities said on Saturday they had no solid leads on the identity of a masked, black-clad gunman who killed a Dallas-area prosecutor in an execution-style slaying in a courthouse parking lot. But law enforcement sources have acknowledged that Thursday's killing in Kaufman, a town of about 6,800 people on the southeastern outskirts of Dallas, may have been an act of retaliation by a drug cartel or other organized crime group, including the white supremacist Aryan Brotherhood. ...