GLENDALE, Ariz. — Luke Air Force Base officials say they have ended altitude restrictions on their F-35 fighter jets put in place after several pilots reported symptoms of oxygen deprivation.

The 25,000-foot (7,600-meter) limit implemented on June 20 came after an 11-day grounding of the new stealth fighters at the training base west of Phoenix. Five pilots reported symptoms between May 2 and June 8.

Brig. Gen. Brook Leonard says in a Wednesday statement that the Air Force hasn’t found a cause for the Luke pilots’ symptoms. But it has boosted training and worked to reduce potential causes for labored breathing or carbon monoxide ingestion. Leonard commands the base’s 56th Fighter Wing.

The military has had a series of problems with various aircraft in recent years involving pilots suffering from lack of oxygen.

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