GLENDALE, Ariz. (AP) — Luke Air Force Base will resume flights of F-35 fighter jets Wednesday after experts and engineers investigated a series of events in which pilots reported symptoms of oxygen deprivation, base officials said Monday.

Officials with the 56th Fighter Wing at Luke said no specific root cause for the events was identified. But specific concerns were eliminated as possible causes including maintenance and aircrew flight equipment procedures.

"We are confident that this initial step with the criteria our team developed will allow us to return to flying F-35s safely," Brig. Gen. Brook Leonard, the 56th Fighter Wing commander, said in a statement.

F-35 flights at Luke have been grounded since June 9 after five separate in-flight incidents involving reports of hypoxia since May 2.

According to the Air Force, the F-35's backup oxygen system in each instance worked as designed and the pilot was able to land the plane safely.

Luke officials said five criteria are being applied to flights as technical and human performance data continues to be gathered.

Those criteria are:

— Avoid the altitudes in which all five hypoxia events occurred

— Modify ground procedures to mitigate physiological risks to pilots

— Expand physiological training to increase understanding between pilot and medical communities

— Increase minimum levels for backup oxygen systems for each flight

— Offer pilots the option of wearing sensors during flight to collect airborne human performance data

Luke, located 15 miles west of Phoenix, is one of the Air Force's primary F-35 pilot-training bases.