A Winthrop auxiliary police officer was arrested late Thursday night by the FBI on charges of selling cocaine from a police cruiser while in uniform, Winthrop police said Monday. Bledar Naco, 34, of Winthrop was arraigned in US District Court in Boston Friday on charges of distribution of a Class B substance, police said, and is being held without bail pending a detention hearing. He did not enter a plea. The arrest followed an investigation by the FBI and Winthrop and Revere police, Winthrop Chief Terence Delehanty said. Auxiliary officers are volunteer members of the force, Delehanty said.