"Undocumented" modifications made to a vintage plane caused it to slam into spectators at a U.S. air show, killing 11 people including the pilot, accident investigators concluded.

The elderly pilot would have fainted due to extreme gravitational force as the 1944 P-51D Mustang lost control, flying faster than it had ever done before in the tragedy last year in Reno, Nevada.

The "experimental" single-seat called The Galloping Ghost was traveling at about 445 knots, or 512 miles per hour, in the third lap of a six-lap race when the accident happened, said the National Transportation Safety Board (NTSB) on Monday.

"During the upset sequence, the airplane's vertical acceleration peaked at 17.3 G, causing incapacitation of the pilot. Seconds later, a section of the left elevator trim tab separated in flight.

"The airplane descended and impacted the ramp in the spectator box seating area, killing the pilot and 10 spectators and injuring more than 60 others," it added.

The plane was flying in the National Championship Air Races on September 16 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 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.

The NTSB report said the plane "had undergone numerous undocumented modifications" to increase speed, including shortening of the wings, modifying the pitch trim system and changing horizontal and vertical stabilizers.

Specifically, deteriorated locknut inserts found in the highly modified (plane) allowed the trim tab attachment screws to become loose, and even initiated fatigue cracking in one screw.

"This condition ... ultimately led to aerodynamic flutter at racing speed that broke the trim tab linkages, resulting in a loss of controllability and the eventual crash," it added.

NTSB chairman Deborah A. P. Hersman said: "In Reno, the fine line between observing risk and being impacted by the consequences when something goes wrong was crossed.

"The pilots understood the risks they assumed; the spectators assumed their safety had been assessed and addressed."