Luke Air Force Base in Arizona is keeping its F-35 fighter jets grounded as it continues to investigate a series of incidents in which pilots reported symptoms of oxygen deprivation.
The F-35s were grounded Friday due to five separate in-flight incidents since May 2.
In each case, the jet's backup oxygen system worked as designed and the pilot was able to land the plane safely.
The Air Force originally intended to resume flight operations Monday at Luke.
But Luke officials say the safety of the pilots is paramount, the base will take as much time as necessary to find the root cause of the incidents and it's unclear when flight operations might resume.
Luke, located 15 miles west of Phoenix, is one of the Air Force's primary F-35 pilot-training bases.