A highly modified vintage aircraft was pushed beyond its structural limits before it crashed into spectators at last year's National Championship Air Races in Reno, according to the findings being presented to the National Transportation Safety Board on Monday in Washington.

Loose screws on a crucial tail assembly are also blamed in the September 2011 crash that killed 74-year-old pilot Jimmy Leeward and 10 people on the ground and injured more than 70.

This year's Reno Air Races start Sept. 12.

The five-member NTSB has already issued several preliminary reports and recommendations for the Federal Aviation Administration and race organizers to improve safety.

NTSB officials said Leeward's World War II-era P-51 Mustang fighter reached 530 mph before it pitched skyward and then slammed nose-first into rows of box seats.

The five-member panel is not expected to call for restrictions on the event. The board has already issued several preliminary reports and recommendations for the Federal Aviation Administration and race organizers to improve safety. A report issued last Tuesday focused on loose screws in the tail of the World War II-era P-51 Mustang fighter that Leeward had modified to increase speed.

A spokesman for the air races said last week that event officials wouldn't comment on NTSB reports ahead of the board's findings on the cause of the crash. But he said event officers were improving preflight inspections.

Leeward, a former stunt pilot and Reno Air Races veteran who lived in Ocala, Fla., named his vintage aircraft the Galloping Ghost.

NTSB officials say he reached 530 mph during a qualifying race before the aircraft pitched skyward, rolled upside down and slammed nose-first into the tarmac amid rows of box seats. Debris and body parts were scattered for more than an acre.

Photos showed a tail stabilizer falling from the plane during the steep climb.

NTSB Chairman Deborah Hersman said in April the climb exerted sudden high gravitational forces that appeared to have incapacitated Leeward as blood rushed from his brain. Instruments aboard the aircraft showed the acceleration topped nine times the force of gravity — well above a 5G threshold at which people begin to experience loss of consciousness.

In addition to loose screws, a preliminary NTSB report on Tuesday focused on the possibility that a sudden gust of wind or turbulence from a nearby plane buffeted Leeward's aircraft just before the crash. The report said a definitive conclusion wasn't possible.

Leeward's age and physical condition weren't considered significant factors in the crash.

Alcohol detected in Leeward's remains by the Washoe County medical examiner's office were attributed to aviation fuel contamination, and NTSB investigators noted that spectators' alcoholic beverages were scattered around the crash crater.

The air race championship, entering its 49th year, is the only event of its kind in the world. It draws thousands every year to Reno Stead Airport, where it features aircraft flying at speeds of over 500 mph sometimes wingtip-to-wingtip around an oval pylon track.

The crash spawned civil liability lawsuits against the pilot's family and mechanics and the host organization.

Some critics called for ending the event, but organizers pressed forward with plans for this year's races amid promises that most NTSB safety recommendations would be implemented.

The National Air-racing Group Inc. and the Reno Air Racing Association Inc. balked at requiring pilots to wear flight suits designed to mitigate high gravitational forces. They said the expensive suits might prove too bulky for cramped cockpits and too hot during competition.

Insurance premiums jumped from $300,000 last year to $2 million this year, but the Nevada state tourism commission last month approved a $600,000 sponsorship to help the Reno Air Racing Association Inc. meet a Sept. 1 deadline for its final insurance payment.