The Second World War-era plane that plummeted into an air-race crowd like a missile was rebuilt for speed, it has emerged.
The 65-year-old Galloping Ghost underwent years of massive overhauls that took 10ft off its wingspan. The ailerons - the back edges of the main wings used to control balance - were cut from about 60ins to 32ins.
Pilot Jimmy Leeward, 74, had said the changes made the P-51 Mustang faster and more manoeuvrable, but in the months before Friday's crash in Reno, Nevada, even he was not certain exactly how it would perform.
"I know it'll do the speed," he said in a podcast uploaded to YouTube in June. "The systems aren't proven yet. We think they're going to be OK."
Investigators do not yet know what caused the plane to pitch sharply into the crowd at the National Championship Air Races, killing nine people, including former Hollywood stunt pilot Mr Leeward, and injuring dozens.
They have focused on the elevator trim tab - a piece of the tail that helps the aircraft maintain lift, which appeared to break off before the crash.
In the highly-competitive, bravado-filled world of air racing, pilots go for broke on the ground and in the sky, hitting speeds of 500mph. Mr Leeward is the 20th pilot to die at the air races since they began 47 years ago, but Friday's crash was the first in which spectators were killed.
"Pilots are a special breed of confident, intelligent, driven perfectionists," said Ken Quick, a commercial airline pilot and a crew member for one of the teams that raced on Friday. "They know what they do is dangerous and demanding, and they eagerly embrace both."
Officials said 69 people were treated at hospitals, including 36 who have been released. Six remained in a critical condition.