WASHINGTON — The U.S. military has grounded its fleet of F-35 fighter jets, built by Lockheed Martin in Fort Worth, as it continues to investigate a runway fire involving one of the planes at Eglin Air Force Base in Florida.In a statement issued late Thursday, the Pentagon’s press secretary said Air Force and Navy officials ordered the F-35s grounded after initial findings on the incident, which occurred June 23.The Air Force halted flights of its F-35s immediately after the fire, and the new action affects the Navy and Marine Corps versions of the jet. Engine inspection results will determine when the jets can fly again, according to the statement.The actions are another blow to the F-35, the costliest U.S. weapons system, which has endured technical problems and cost overruns during its development. More than 6,000 people work directly on the F-35 program at Lockheed’s complex in west Fort Worth, which employs about 13,300 overall. More than 100 F-35s have been delivered to the U.S. military.“Lockheed Martin is working closely with the F-35 Joint Program Office and industry partners in supporting” the investigation, company spokesman Gordon Johndroe said in a statement. “Any further questions should be directed to the Joint Program Office.”The grounding puts in question whether the F-35 will still be able to fly at this month’s Farnborough International Airshow for the first time. A decision will be made early next week on whether the F-35 will participate at Farnborough and at the Royal International Air Tattoo in the U.K. this month, the Pentagon said in its statement. The scheduled appearance at Farnborough during the week of July 14, which attracts buyers from around the world, is seen as a key to drum up more international sales of the F-35, which are needed to boost production and lower its average cost.About five years behind schedule, the F-35 has been plagued by a costly redesign, bulkhead cracks, excessive weight and delays in software. The projected cost of building more than 2,400 F-35s has increased to about $400 million, a 71 percent increase in inflation-adjusted dollars since the contract with Lockheed was signed in 2001. The fire at Eglin broke out at the rear of the plane while the F-35 was preparing to take off on a training mission. The pilot aborted the takeoff and emergency responders moved in and extinguished the fire with foam, according to the Air Force. The pilot was uninjured, officials said.The Pentagon says that additional inspections of F-35 engines have been ordered, and that the return to flight of the F-35 fleet will be determined based on inspection results and analysis of engineering data. This report includes material from The Associated Press, Bloomberg News and the Star-Telegram archives.