Some F-35 joint strike fighters have been grounded for a new round of inspections after worries about their fuel systems surfaced this week, the Pentagon’s Joint Program Office said on Thursday.

All of the aircraft were grounded earlier this month due to a fuel tube issue, and now two more engine fuel tubes need to be inspected in F-35s with higher flight hours, the JPO said.

“The inspection maintains F-35 fleet safety standards as older engines may require fuel tube replacement. The procedure to inspect and replace can be done by flightline maintenance without removing the engine,” the office said in a statement.

All F-35s that have not reached the threshold of flight hours will remain in operation.

The earlier grounding on Oct. 11 and subsequent inspections were prompted by the crash of a Marine Corps F-35B in South Carolina in September.

“The joint government and industry technical team has completed their assessment of the fuel supply tubes within the Pratt & Whitney engine on F-35 aircraft,” the JPO said. “In addition to the previously identified failed tube, the analysis has identified two additional fuel supply tubes that require inspection.”