EAST HARTFORD, Conn. (AP) — Heat was the cause of a cracked blade in an engine on an F-35 fighter jet that prompted the entire fleet to be briefly grounded last month, manufacturer Pratt & Whitney said Wednesday.

Bennett Croswell, president of the company's military engines division, said on a conference call that finding the cause was "very good news." Heat is preferable to the effects of fatigue on the engine part because fatigue spreads, making the problem worse, he said.

Croswell said an inspection found the 6/10-inch long crack in a turbine blade. The Pentagon grounded its F-35 fleet on Feb. 22 after discovering the crack on a jet at Edwards Air Force Base in California.

"It was prudent to suspend flight operations while we inspected the blade," he said.

What Croswell called "thermal creep" was due to the engine operating longer than usual at a high temperature. It was the only affected engine of about 75, he said.

Pratt & Whitney, based in East Hartford, Conn., is a subsidiary of United Technologies Corp.

Winslow T. Wheeler, director of the Straus Military Reform Project at the Project on Government Oversight, a watchdog group, said in an email that the cracked blade is "one bump in a long, difficult road" for the F-35. He said 30 percent of the initial laboratory testing is completed and more stressful testing has yet to be done.

Only by 2019 when testing is set to wrap up will the cost and performance of the F-35 be known.

The Project on Government Oversight said at the time of the grounding that it was not likely to mean a significant delay in fielding the stealthy aircraft. The group called the F-35 a "huge problem," saying its cost is rising and is already unaffordable and its performance is disappointing.

The F-35 is the Pentagon's most expensive weapons program at an estimated cost of nearly $400 billion. The Pentagon envisions buying more than 2,400 F-35s but some members of Congress are balking at the price tag.