A mechanical failure in the rotor blades was blamed Wednesday for the fiery desert crash of an Air Force Reserve helicopter that killed all 11 Green Berets and four crewmen aboard, a military spokesman said.

Maj. Fran Tunstall, a Defense Department spokesman, said the March 12 crash in a farm field in Avra Valley resulted from the failure of one of the five "sleeve-and-spindle" devices that attach the helicopter's rotor blades to the hub.As a result of the investigation, all H-3 Jolly Green Giant helicopters will remain grounded until inspections of the assemblies are completed, a process that could take six to 10 weeks, Tunstall said.

The craft that crashed was among six assigned to the 71st Special Operations Squadron, an Air Force Reserve unit at Davis-Monthan Air Force Base. The H-3 went down while ferrying Army Special Forces Green Berets to a training site near Gila Bend in central Arizona.

Eleven Green Berets based at Fort Bragg, N.C., and four Air Force personnel stationed at Davis-Monthan in Tucson were killed.