A U.S. Air Force F-35A Lightning II Joint Strike Fighter navigates the aircraft toward an Air Force KC-135 Stratotanker on May 16, 2013, off the coast of Northwest Florida. (U.S. Air Force photo by Master Sgt. John R. Nimmo, Sr./RELEASED)

Officials investigating the F-35 fire that temporarily caused the fleet of fighter planes to be grounded this summer should determine the “root cause” by the end of the month and should have a fix for the planes by the end of the year, officials said Monday.

Since the fire on June 23, investigators have determined that there was excessive rubbing between a seal that’s part of an engine rotor and a piece of hard plastic designed to absorb the contact. Because the engine expands when it gets hot and shifts during flights — especially during severe maneuvers — some rubbing between the parts is expected.

But speaking to reporters at the annual Air Force Association Conference at National Harbor, Air Force Lt. Gen. Christopher Bogdan said, referring to the amount of rubbing: “Quite frankly it was underestimated. It’s as simple as that.”

The rubbing caused the temperature of the metal to rise to 1,900 degrees, Bogdan said, well above the normal 1,000 degrees. That caused micro-cracking in the engine, which eventually grew.

In the weeks before the fire, the F-35 had been through a series of maneuvers that were aggressive but all within the plane’s capability, he said.

“That’s why we’re worried,” Bogdan said. “If it were outside the envelope we’d say just don’t fly there but it was inside the envelope, so that’s why we have to fix it.”

Still, he said, the incident shouldn’t cause any significant delays to the project. The F-35 is made by Bethesda-based Lockheed Martin.

The cost of fixing and retrofitting any other planes would be paid for by the engine’s manufacturer, Pratt & Whitney. Bennett Croswell, Pratt & Whitney’s president of military engines, said that it would “not require a significant teardown of the engine,” and that it “would not be hugely expensive.”

A fix could be in place by the end of the year, he said.

“While we don’t like to have these events, they do happen in a program like this,” he said.

The fire occurred during a training mission at Eglin Air Force Base in Florida when a pilot was beginning to take off. He quickly aborted the flight and got out safely. But Bogdan said Monday that the incident could have been much worse had the fire broken out just a little while later.

Still, he said that officials were committed to fixing the problem and that it was not emblematic of the cost overruns and delays that have plagued the program in the past. The program has largely gotten back on track since he took over two years ago, officials have said.

Speaking at the conference Monday, he had sharp words for those who wanted to continue to cite the problems of the past: “Get over it.”