The US Air Force said recently that faulty equipment is likely responsible for F-22 pilots often suffering hypoxia — oxygen deprivation — while flying the stealthy modern fighter jet.

Maj. Gen. Charles Lyon, the official charged with resolving the oxygen problem, told the Associated Press that the problem was caused by “previously unknown restrictions on pilots’ breathing.”

“We’re not ready to declare victory yet,” Lyon said.

Lyon told the AP that the Air Force has narrowed down the problem to two issues: malfunctioning pressure vests worm by pilots and leaky hoses and hose connectors on the life support systems.

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The pressure vests, or “G-force vests,” were filling with air when they should not have been, making it harder for pilots to breathe. The Air Force took the vests out of service and will fix them, Lyon told the AP. This has resulted in a temporary lowering of the F-22’s ceiling, since the pressure vests are supposed to protect the pilots’ lungs if pressure is lost at high altitude.

The Air Force will also fix the faulty hoses and connectors.

The oxygen problem grounded the F-22 fleet for parts of last year after two F-22 pilots broke protocol and complained to “60 Minutes” about the oxygen problem. The pilots were considered “whistle blowers” and were not punished.

The Lockheed Martin/Boeing F-22 “Raptor” is the most advanced production fighter ever built. It is an air superiority fighter with air and ground attack capabilities, considered the next generation beyond the F-15 “Eagle” and F-16 “Fighting Falcon” in the Air Force. The oxygen problem has caused pilots dizziness and other problems after flights.

The Air Force has a fleet of 170 F-22s.