The Air Force is set to make a decision Monday on whether F-35s at Luke Air Force Base in Arizona will return to the sky, said Brig. Gen. Brook Leonard, 56th Fighter Wing commander.

Leonard said the service is looking at altitude restrictions and safety measures that will allow the aircraft to fly, but still has not pinpointed why five F-35 pilots at the base reported oxygen deprivation incidents over the past five weeks. One of the affected pilots belongs to a visiting foreign military.

"If everything is deemed to lead us to safe flight and the pilots have confidence, the soonest we'd be able to return to flying would be Tuesday afternoon," Leonard told reporters Friday. "That's assuming everything goes well."

The Air Force grounded the aircraft last Friday and is the latest service faced with mysterious oxygen deprivation issues in its fighter jets. Its F-35s at the Arizona base comprise 25 percent of the total military-wide fleet, Leonard said.

On Thursday, the Navy also said it still does not know the ultimate cause after its review of spikes in oxygen deprivation incidents with the F/A-18A-D model Hornets and the T-45 training jets, which has grounded student pilots for nearly three months.

Two systems that provide breathing air to pilots and pressurize cockpits remain top suspects for both services.

"Our concern locally was why five and why just at Luke," Leonard said.

The incidents can cause sickness, hyperventilation, anxiety and panic in pilots. The Air Force called a town hall meeting at the Arizona base after the five incidents to assure worried spouses if would maintain safety.

Leonard said the Air Force eliminated possible missteps by crew members and incorrect procedures, but did find the incidents appeared to occur at similar altitudes. The service is now wading through that data on altitude to determine what role it might have played.

"We did not find anything specific but what we were able to do is just to continue to examine what I would call a very complex multi-dimensional human and machine issue," Leonard said. "As well, the solution is going to be a very multi-layered human and machine solution as well."