FORT BRAGG, N.C. (AP) _ An unqualified Air Force pilot who left his retirement party to take a farewell flight was in the co-pilot's seat of a helicopter when it crashed last August, killing all four aboard, an Air Force report says.

The report, issued Monday by the 23rd Air Force, stated that the use of a co-pilot not qualified to fly the UH-60 Blackhawk violated Air Force regulations, but that circumstances surrounding the crash made its cause inconclusive. An investigating officer wrote that the probe was hampered by extensive fire damage.

The co-pilot was identified as Maj. Kenneth Reinhard, 41, of Cibolo, Texas.

Also killed in the Aug. 14 crash were Capt. Ralph Gehringer, 26, of Tacoma, Wash., a pilot; Capt. Jeffrey Stevens, 26, of Billings, Mont., another pilot; and Senior Airman Jeff Beske, 21, of Alexandria, Minn., a flight engineer.

The report doesn't say whether Reinhard was flying the dual-control helicopter, said Rob Fortenberry, a spokesman for Pope Air Force Base, from which the helicopter departed before crashing in woods at adjacent Fort Bragg.

Although Reinhard, who was stationed at Pope, was an experienced pilot and qualified to fly two other types of helicopters, he was not qualified in the Blackhawk, the accident report states.

Forty people have been killed in crashes of Blackhawks since 1978 and the choppers have been grounded at least three times in the last two years because of possible design flaws. The helicopter, made by Sikorsky Aircraft, is used primarily by the Army.