The U.S. Air Force cancelled flying operations of the F-35A Lightning II at Luke Air Force Base, Arizona, last week to investigate the cause of several recent incidents of pilots experiencing hypoxia-like symptoms. Operations remained suspended several days later.

From May 2 through June 9, when the 56th Fighter Wing cancelled local flying operations at the base, five F-35A pilots reported physiological incidents while flying. In each case, the fighter’s backup oxygen system operated as designed and the pilot followed the correct procedures to land the aircraft, according to base officials.

The flight suspension affected 55 Lockheed Martin-built fighters at the base, which is located about 24 miles west on Phoenix. As of the announcement, the cancellation was limited to Luke AFB.

Wing officials planned to brief U.S. and international pilots on the incidents that had occurred and the successful actions pilots performed in response. Medical staff was to brief pilots on physiological symptoms and “extensive measures” that were being taken to analyze data collected from the incidents.

“In order to synchronize operations and maintenance efforts toward safe flying operations we have cancelled local F-35A flying,” said Brig. Gen. Brook Leonard, 56th Fighter Wing commander. “The Air Force takes these physiological incidents seriously, and our focus is on the safety and well-being of our pilots. We are taking the necessary steps to find the root cause of these incidents.”

The Pentagon’s F-35 Joint Program Office (JPO) said it is overseeing a “comprehensive review” of the circumstances of the physiological episodes at Luke AFB.

“The F-35 enterprise takes a multi-disciplinary approach to monitoring and tracking physiological issues within the fleet and integrating findings to improve the weapon system and the tactics, techniques and procedures with which it operates,” the program management office stated. “The ongoing JPO review is being conducted by a joint government and industry team of engineers, maintainers and aerospace physiologists from the JPO, the Air Force Lifecycle Management Center, Naval Air Systems Command, Lockheed Martin, and others.”

According to the JPO, F-35 pilots began training at Luke AFB just over a year after the 56th Fighter Wing, part of the Air Force’s Air Education and Training Command, received its first F-35A in March 2014. Pilots and maintainers from Australia, Norway, Italy, The Netherlands, Japan and Israel train at the base. On May 20, two Japan Air Self-Defense Force pilots became the first pilots from their country to graduate from the F-35 program at the base.

The suspension of F-35A flights at the base comes two months after the Navy temporarily paused operations of the T-45C Goshawk training jet out of concern for physiological episodes caused by oxygen-system contamination. The Navy has seen steadily increasing rates of such episodes reported by the pilots of F/A-18A-D Hornets, F/A-18E/F Super Hornets and EA-18G Growlers. The Air Force encountered similar problems that led it to ground its F-22 Raptor fleet for several months in 2011.