Luke Air Force Base announced last week that it was canceling flying operations of the F-35A Lighting II after a series of incidents where pilots experienced hypoxia-like (oxygen deficiency) symptoms.

According to a release, base officials say there’s been five reported incidents since May 2. But in all cases, the aircraft’s backup oxygen system has “operated as designed” and the pilots landed safely.

Officials with the 56th Fighter Wing at Luke AFB were working to educate other pilots on the situation and hypoxia symptoms while showing them successful actions to take in order to safely recover their aircraft.

As of Monday, according to a statement from a Luke AFB spokesperson, the F-35s remained grounded:

"The 56th Fighter Wing will continue their pause in local F-35A flying to coordinate analysis and communication between pilots, maintainers, medical professionals and a team of military and industry experts. This coordination will include technical analysis of the physiological incidents to date and discussions on possible risk mitigation options to enable a return to flying operations. Updates will be provided as our teams work together toward safely returning to building the future of airpower through trained F-35A pilots. The safety of our Airmen is paramount and we will take as much time as necessary to ensure their safety."

The canceled flying operations are limited to Luke Air Force Base, according to a release.