WASHINGTON (AP) _ Pilots who helped investigate the crash of a V-22 Osprey aircraft that killed four Marines in December say the accident was caused by a design flaw that had been known for months but was ignored, The Washington Post reported Thursday.

The pilots said the flaw in the aircraft's hydraulic system was compounded by a software glitch that could have been detected by more rigorous testing.

The Marine Corps has said the Dec. 11 crash was caused by a failure of the hydraulic system, followed by software errors.

Pilots interviewed by the Post said they believe both problems slipped by because the Marine Corps wanted to win Pentagon funding for full production of the plane.

The pilots, who are participants in a military ``mishap board,'' said they were ready to speak out because they increasingly distrust the Corps' leadership and worry that some of their conclusions might be omitted in a public report on the accident being released Thursday.

After the crash, the Corps grounded its initial fleet of eight remaining Ospreys and the defense secretary appointed an independent panel to review the program.

The release of the accident report comes just as the review panel, headed by retired Marine Gen. John R. Dailey, is preparing its findings and recommendations, which could decide the fate of the Osprey.

The Osprey is unique in its ability to take off like a helicopter, rotate its propellers 90 degrees and fly like an airplane. The Marine Corps has plans to buy 360 Ospreys to replace its fleet of aging CH-46 and CH-53 transport helicopters.

A decision to produce the plane was postponed after two Osprey crashes last year killed 23 Marines, raising questions about the safety of the aircraft.

A Marine spokesman, Maj. Patrick Gibbons, told the Post the Corps would not comment on the pilots' allegations while the accident report was pending.