(Reuters) - Extensive modifications to a racing airplane -- nearly all of them undocumented -- led to mechanical failure that probably caused the deadly crash at a Reno air show last year, U.S. transportation officials ruled on Monday.

The Galloping Ghost, a highly modified, World War II-era P-51 Mustang fighter plane, lost control while traveling at 445 knots and crashed into the stands on September 16, killing the pilot and 10 spectators. Another 64 people were injured, 16 of them seriously.

By a 5-0 vote the National Transportation Safety Board adopted the conclusion reached by a panel of experts whose investigation was aided by hundreds of photographs and a dozen videos taken by spectators at the Reno Air Races.

The board recommended changes that can be implemented in time for this year's show set for September 12-16. The Reno course should also be changed and reinforced barriers installed in the grandstands to improve spectator safety, the NTSB said.

Hollywood stunt pilot Jimmy Leeward, 74, who owned the Galloping Ghost, made extensive modifications to the plane to improve its speed and failed to report all but one of them to the Federal Aviation Administration as required, investigators told the NTSB at Monday's hearing in Washington.

While in third place on the third lap of a six-lap unlimited class race, Leeward unexpectedly rolled to the left, then to the right and climbed as a piece of the tail section called the left elevator trim tab broke off, according to the presentation given to the board.

Leeward was "incapacitated" by 17 gravity forces during the unusual movements, far more than humans can stand and well beyond the maximum 3.5 G forces expected in the race, the board was told.

"At the heart of this tragedy was the fatal intersection in transference of risks from participant to their observers. One moment, spectators were thrilled at the spectacle of speed only to have it followed by inescapable tragedy," said Deborah Hersman, chairman of the NTSB.

"The pilots understood the risks they assumed. The spectators assumed that their safety had been assessed," she said.

At 445 knots, the plane was traveling 35 knots faster than it had ever flown before, investigators said.

"Contributing to the accident were the undocumented and untested major modifications to the airplane from the pilot's operation in the racing environment without testing," the NTSB found.

The NTSB recommended that participants in the race submit planes to an engineering analysis to determine if any aircraft modifications are safe.

(Reporting by Daniel Trotta; Editing by Paul Thomasch and Cynthia Osterman)