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 including Hutchinson High School graduate Cheri Elvin. The crash also injured 4 other family members.

Elvin was visiting the Reno Air Races with her Husband, Chuck, a 1955 graduate of Haven High School, their sons Bill and Brian Elvin and their daughter-in-law Linda Elvin, when the plane lost control and crashed into the grandstands. Chuck, Bill, Brian and Linda were among the 70 hurt in the crash. 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.