Details of the investigators' final report were to be released later in the day at a Pentagon news conference.
The pilot's too-rapid approach produced a phenomenon known as "vortex ring state" essentially a stallthe investigators concluded. The Osprey plunged into a fatal nose dive, killing the crew of four and their 15 passengers.
The crash was the worst aviation disaster for the Marines since 22 were killed in a helicopter crash in South Korea in 1989.
The Marine Corps halted flights of its remaining Ospreys after the April 8 accident, then resumed flying in early June.
The Marine Corps, which had previously announced that investigators found no evidence of mechanical, engineering or structural flaws in the Osprey, ascribed the cause of the crash to "human factors." Officials said they would not label it as "pilot error," although they acknowledged that the aircraft commander, Maj. John Browconsidered one of the Marines' most skilled pilotscommitted mistakes.
Brow brought the Osprey into its approach to Marana airport at a rate of descent far in excess of the plane's maximum safe descent rate of 800 feet per minute, officials familiar with the investigation have said. What remained a mystery is why Brow did this. The plane had no cockpit voice recorder.
Because of the rapid rate of descent and a slow forward air speed, the Osprey lost lift, investigators previously disclosed. It was less than 300 feet off the ground when it flipped to the right and plunged nose first into the ground.
The Osprey is unique in its ability to take off like a helicopter, rotate its propellers 95 degrees and fly like an airplane. A group of four Ospreys were participating in a mock "noncombatant evacuation" exercise at Marana, about 30 miles northwest of Tucson, when Brow's aircraft crashed and burned.
The manufacturersBoeing Co. and Bell Helicopter Textronare due to deliver 11 Ospreys to the Marines this year. Eventually the Marines are to field 360 of them to replace the corps' Vietnam-era CH-46 helicopters as the primary means of transporting troops into combat from ships offshore.
The Air Force and the U.S. Special Operations Command are to buy a smaller number of Ospreys.