An elderly pilot whose modified 1944 aircraft slammed intospectators last year at a Reno, Nevada airshow likely fainted due to extreme gravitational force brought on by high flight speeds, officials said Tuesday.

The preliminary results of an investigation by the National Transportation and Safety Board found that a series of racing modifications were made to the aircraft, which reached at least 9-G's before crashing into the crowd below, killing 11 people including thepilot and injuring more than 60 people.

"Our investigation found that this pilot in this airplane had never flown this fast on this course," said NTSB chair Deborah Hersman.

"There was a very clear upset here and a loss of control," she added. "It appears very likely that this pilot was incapacitated at the immediate onset of this upset."

The vintage P-51 Mustang was flying in the National Championship Air Races on September 17 last year when its elderly pilot, a race veteran, apparently lost control of the aircraft and it plunged at full speed into spectators.

Amateur video captured the moment the plane, a single-seat fighter aircraft called the "Galloping Ghost," barrel-rolled wildly through the sky and smashed at a near-vertical angle into a roped-off area for spectators, narrowly missing a grandstand packed with many more people.

"We know that there were extensive modifications made for racing over the years," Hersman said of the aircraft, including an eight-foot (2.4 meter) reduction in wing span bringing it from 37 to 29 feet (11.2 to 8.8 meters).

The investigation also showed a bit of the tail known as the left elevator trim tab came loose about six seconds before the crash.

The speed of the aircraft made it exceed the 9-G limit on its internal monitor, known as an accelerometer, she said.

The average person would likely pass out in 4 to 5-G conditions as blood pools in the lower extremities and the brain is deprived of oxygen, experts say.

Officials will pinpoint a more precise probable cause when the NTSB issues its final report in the coming months, she said.

The NTSB also issued series of seven recommendations for making air shows safer.

"We are not here to put a stop to air racing," said Hersman. "We are here to make it safer."