Another pilot in Arizona experienced lightheadedness and breathing difficulties this week, Arizona Central reported Thursday.

The event took place on Monday, a spokesman told AZ Central, which is owned by USA Today. Last month, several squadrons of F-35 jets at Luke Air Force Base in Glendale, Arizona, were grounded following multiple episodes of hypoxia in the month of May. The oxygen flow regulators weren’t fixed, investigators didn’t know why they had malfunctioned, but the military decided to continue flying the jets anyway while safety risks loomed.

Experts were tapped to help figure out what was going on but the service has next to no idea why hypoxia is still occurring among pilots in the $1.5 trillion jet program. “No specific root cause for the physiological episodes was identified during recent visits from experts and engineers from the Joint Program Office, Lockheed Martin, the Air Force Research Laboratory,” the US Air Force 56th Fighter Wing Public Affairs office said in June.