The engine fire that prompted the U.S. military to ground its entire F-35 Joint Strike Fighter fleet was caused by “excessive” rubbing of fan blades in the jet’s engine, the Pentagon’s top weapons buyer said on Sunday.

“From the evidence we have so far, we don’t see at this point what I would call systemic difficulties,” Defense undersecretary Frank Kendall told reporters in London. “We’re not noticing it throughout the fleet.”

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The Marine Corps variant of the F-35 was due to make its international debut at the world-renowned Farnborough International Airshow in Hampshire, England, which formally kicks off on Monday.

That was before a June 23 blaze erupted on an Air Force version of the jet as it took off from Eglin Air Force base in Florida. The Pentagon grounded all 97 existing planes on July 3.

Kendall said the fire was caused by “rubbing between some blades of the engine and the cowl around them.”

Lockheed Martin manufactures the jet and Pratt & Whitney builds its engines.

Kendall did not say when the fleet-wide flight ban would be lifted, as inspections are ongoing. Expo organizers have already confirmed the fighter would not appear Monday, but held out hope it could fly later in the week.

At almost $400 billion, the F-35 is the most expensive weapons effort in U.S. history. Already seven years behind schedule, project costs have ballooned by roughly 70 percent even though the U.S. has reduced the number of planes it intends to purchase by more than 400 to roughly 2,400 aircraft.