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.Nicholas Jacques can be reached at email@example.com.