The U.S. military is grounding some F-35 aircraft after a pilot ejected from the Marine version of the Lockheed Martin jet during a crash earlier this month in Fort Worth. The F-35 Joint Program Office said in a statement to NBC DFW that it was grounding an undisclosed number of aircraft “which have been evaluated to be of higher risk from flight operations while the investigation into the mishap on December 15 continues.”

The jets would be grounded “until procedures can be developed for their return to flight,” the statement continued.

The JPO did not say how many planes were unable to fly.

The update comes about two weeks since a pilot ejected from a Lockheed Martin F-35B Lightning II fighter jet during a failed landing at the Naval Air Station Joint Reserve Base Fort Worth. The pilot safely ejected from the airplane and was hospitalized as a precaution.

At the Pentagon, a spokesman said that the aircraft was being flown at the time of the crash by a U.S. government pilot, although it had not been transferred to the military yet by manufacturer Lockheed Martin.

An investigation is underway to determine what caused the crash.