(CNN) -- The Pentagon's most expensive weapons program ever, the F-35 warplane, is grounded again.
Developed at a cost of nearly $400 billion so far and beset for years by cost overruns and delays, the so-called Joint Strike Fighter was put down temporarily this week following a runway fire at Eglin Air Force Base in Florida. No one was hurt.
Pentagon spokesman Rear Adm. John Kirby said the cause of the June 23 incident remains under investigation, and the timing of when they might take off again is unclear.
"Additional inspections of F-35 engines have been ordered, and return to flight will be determined based on inspection results and analysis of engineering data," Kirby said.
Engine maker Pratt & Whitney said in a statement that it was working with Air Force investigators to inspect all engines in the fleet. It declined further comment.
Lead contractor Lockheed Martin also said it was working with investigators. It is producing variants of the plane for the U.S. Navy, Marines and Air Force.
The Pentagon wants more than 2,400 ultimately, while hundreds more are expected over time to go to allies such as South Korea, Japan and Australia.
More than 100 planes have been built so far, most for testing, but the program is still in its development and training phases.
The military says the stealthy fighter will be "the most affordable, lethal, supportable and survivable aircraft ever to be used" by so many services worldwide.
But its production has been controversial for its soaring cost history -- the price tag has nearly doubled from early estimates, to $135 million per unit as of last year, according to a U.S. Government Accountability Office report in March -- as well as its schedule, software and other setbacks. Test flights began in 2007.
While all models have been grounded in the past for various problems, the version for the Marines, the F-35B, has had more issues. Experts say that's mainly due to its design for shorter takeoffs and vertical landings.
Richard Aboulafia, a Teal Group analyst, said every component of the F-35 overall "is pushing the frontiers of technology" as engineers combine extraordinary engine power with a lighter weight design.
He said the program has made gradual progress in recent years where glitches get resolved pretty quickly compared with 18 to 24 months ago, when setbacks seemed to come one right after another.
"It's a tremendously complex project," he said, adding that cost issues remain a concern.
The F-35 is due to make a much-anticipated appearance at the international air show at Farnborough airport in Hampshire, England, in coming weeks. Aboulafia said a no-show could indicate a more serious problem with the engine.
CNN's Dana Ford and John Crawley contributed to this report.