At about 1220 mountain time, Albuquerque Center lost radar and radio contact with a Piper PA-23-250 near Flagstaff. At the time of the report, the airplane had not been found and the four occupants are presumed dead. The airplane was flying VFR from Boulder City, Nev., to Midland, Texas, when the pilot reported he could not maintain VFR. He was given vectors to Flagstaff and reported the airport in sight, then reported losing visual contact. He then requested an IFR clearance and was told to climb and hold over the Flagstaff VOR. No further transmissions were received.