The U.S. military said late Thursday that it had grounded the entire fleet of 97 Lockheed Martin F-35 fighter jets until completion of additional inspections of the warplane’s single engine.

The Pentagon’s F-35 program office, the Air Force and the Navy issued directives on Thursday ordering the suspension of all F-35 flights after a June 23 fire on an Air Force F-35A jet at Eglin Air Force Base in Florida, according to statements by the Pentagon and the F-35 program office.

The Pentagon said U.S. and industry officials had not been able to pinpoint the cause of the fire, which occurred as a pilot was preparing for takeoff. The pilot was not injured.

The Pentagon said preparations were continuing for F-35 jets to participate in two U.K. air shows later this month, but a final decision would be made early next week. The fire already has derailed plans for an F-35 jet to fly by a naming ceremony for Britain’s new aircraft carrier on Friday.

The June 23 incident was the latest to hit the Pentagon’s costliest weapons program, the $398.6 billion F-35 Joint Strike Fighter. It followed an in-flight oil leak that triggered mandatory fleetwide inspections of the jets last month.

“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,” the Defense Department said in a brief statement.

The F-35’s engine was built by Pratt & Whitney, a unit of United Technologies. Pratt & Whitney said late Thursday that it was working closely with Air Force officials investigating the fire to determine the cause of the incident and to inspect all engines in the fleet. Spokesman Jay DeFrank said it would be inappropriate to comment further since the incident was the subject of an investigation.

The Pentagon’s F-35 program office said that determining the root cause of the fire and potential mitigating actions were its highest priority. It said that effects on the flight testing, training and operations of the radar-evading warplane were being assessed.

Air Force Lt. Gen. Christopher C. Bogdan said the F-35 program office would continue to work closely with the military services and industry as engineering findings were analyzed, and would participate in decisions about resuming F-35 flights.

“We will contribute to the return to flight determination, and will aim to do what is prudent for the enterprise at large without compromising the ongoing mishap investigation,” Bogdan said.

Reuters reported Wednesday that U.S. and British authorities were preparing directives ordering a mandatory engine inspection estimated to take about 90 minutes.

British officials remained part of the discussions with U.S. officials and agreed with the U.S. recommendation to ground the jets, pending further inspection results, the F-35 program office said Thursday.

The Royal International Air Tattoo (RIAT) kicks off on July 11 followed by the Farnborough International Air Show starting on July 14.

— Reuters