The plane that plummeted into the crowd at the Reno air race in America was a Second World War-era machine that had been rebuilt for speed.

The 65-year-old "Galloping Ghost" underwent years of overhauls that took a full 10 feet off its wingspan. The ailerons  – the back edges of the main wings used to control balance  – were also cut from about 60 inches to 32 inches.

The changes made to the P – 51 Mustang made it faster and more maneuverable, according to the pilot, Jimmy Leeward, who died in the crash. He admitted in a podcast uploaded to YouTube that he didn’t know how it performed before Friday’s crash.

"I know it'll do the speed," he said in June. "The systems aren't proven yet. We think they're going to be OK."

In the podcast, Leeward called some of the changes "extremely radical," compared some to systems on the space shuttle and explained that he had increased the plane's speed capabilities to be more like those of a modern fighter jet.

Investigators don't yet know what caused the plane to pitch sharply into the crowd at the National Championship Air Races in Reno, killing nine people and injuring dozens.

But they are looking into a lead that the plane was equipped with an outward-facing video recorder that would have streamed real-time data to the racing team before the crash.

They have also focused on the "elevator trim tab" – a piece of the tail that helps the aircraft maintain lift and appeared to break off before the crash.

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.

National Transportation Safety Board spokesman Terry Williams said his agency would look into the oversight of modifications to Leeward's plane as part of its investigation.

"We're not saying they did something right or wrong in this accident," Mr Williams said. "We look at all angles in every accident investigation we do."

Meanwhile, federal investigators recovered memory cards that could be from recording devices on the downed plane.

They also said they had found no indication yet that the pilot of the plane sent out a distress call before his sleek silver jet plunged nose-down into the tarmac.

Watch the crash here: