The Air Force still hasn't determined what is causing pilots to experience oxygen deprivation symptoms at Luke Air Force Base, an official said Friday.

The revelation comes one week after the service grounded all 55 F-35A Lightning IIs at the Arizona base and other previously undisclosed incidents have come to light.

"We learned a lot from each other" over the past week, Brig. Gen. Brook Leonard, 56th Fighter Wing commander, told reporters during a call Friday, referencing ongoing maintenance and strategic initiatives the base has been working on since operations halted June 9.

But "we did not find a specific cause that we could put our finger on and just fix and then all of a sudden return to flying with safe operations," said Leonard, an F-35 instructor pilot and former F-16 Fighting Falcon pilot.

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The 56th Fighter Wing halted operations for all F-35As last week after pilots complained of hypoxia-related issues.

But the base hopes it can return its F-35s to flight by Tuesday, Leonard said. Engineers, maintainers and aeromedical specialists, including officials from the aircraft's manufacturer, Lockheed Martin, have been on the case, he said.

Without giving specifics, Leonard said, "We did eliminate a lot of areas" of what the causes could be regarding the maintenance or aircrew management side of operations that could cause physiological incidents.

"The solution is going to be a very multi-layered human and machine solution," he said. "We're progressing toward that."

On Thursday, Air Force officials disclosed to Military.com that there have been 15 reported F-35A in-flight and ground physiological events since April 2, 2011, including the most recent events at Luke.

Leonard on Friday added that within that same timeframe, 23 cases have occurred across the Joint Strike Fighter fleet -- 15 Air Force models, three Marine Corps B models and five Navy C models. He said the other services' incidents occurred "some years back."

In the 23 cases, 13 -- across all three models -- were discovered to have a root cause, the commander said.

A defense official told Military.com on Friday those 13 were plagued by "pressure issues, contaminated oxygen and ground events." Ground events, for example, could be attributed to "lots of jets on a ramp, of which exhaust fumes may have played a role," the official said.

The remaining 10 remain unexplained. "We may never arrive at a determination for some of these," the official said.

As for recent events, which are limited to Luke, "Some are specific to the person," the defense official said. "How they were flying, at what altitude they were flying," among other variables could play a role, the official said.

Leonard said the hypoxia-like symptoms at Luke were linked to pilots flying at a particular altitude, but would not disclose specifics.

The general said the Air Force hasn't ruled out that the issue could be linked to the On-Board Oxygen Generating System, known as OBOGS.

"We do think the OBOGS system is not as robust as it can be; however, according to all our testing, it meets the minimum standard," he said.

There are plans to modify the system, made by Honeywell, Leonard said, but he would not give details. "While that could be weakness in the OBOGS system … we did not find that a causal across the five instances," he said.

Of the recent incidents, four jets belong to the U.S. Air Force and one to an international partner, Leonard said. Australia, Norway and Italy are currently training alongside the service at the base.

Avoiding Flight Envelopes

The Air Force has created initiatives to keep pilots safe when flying, and to avoid experiencing symptoms -- shortness of breath, confusion, wheezing -- in-flight, Leonard said.

"We're looking to avoid the flight envelopes that all of the flight aircraft were in when they occurred," he said, referencing a temporary, but undisclosed, altitude restriction.

The service also is exploring wearable sensor technologies that "allow us to tell the oxygen level in the pilot, similar to what we did in the F-22" [Raptor], Leonard said.

Other measures include limiting pilots' exposure to ground effects -- by limiting how long they are exposed to high heat on the ground, excess exhaust or traces of jet fuel before getting into the cockpit, for example -- and instructing pilots how best to identify symptoms before an event occurs.

"Last but not least, we're making sure our backup oxygen systems … each time an aircraft takes off, those systems are as full and capable as they can be," Leonard said.

The defense official added, "We don't want to relearn F-22 issues here."

There have been no additional reports from F-35A pilots suffering similar symptoms, Air Force spokesman Capt. Mark Graff recently said.

'Complexity Tends to Hide Bugs'

The issue of military pilots suffering hypoxia-like symptoms in-flight isn't limited to the F-35 fleet.

Pilots flying the F-22 fifth-generation stealth jet experienced hypoxia symptoms on occasions between 2008 and 2012. One pilot died as a result, and one had a near-death scare, with dozens more pilots experiencing confusion and disorientation while flying, according to an ABC News investigation at the time.

Then-Pentagon spokesman George Little said investigators found the cause to be a faulty valve in the high-pressure vest worn by pilots at extreme altitude, which was restricting their ability to breathe.

More recently, the Navy went so far as to equip the aircraft carrier USS George H. W. Bush with specialized equipment called a transportable recompression system, or hyperbaric chamber, amid a review of physiological episodes affecting pilots who fly the T-45 Goshawk trainer and the F/A-18 Hornet.

Dan Ward, a former engineering officer and retired Air Force lieutenant colonel, in a 2015 opinion piece for the website "War is Boring," wrote that, like its stealth jet predecessors, the F-35 will likely experience similar hypoxia setbacks.

"Forget the enormous cost overruns. Excuse the epic schedule delays. Overlook the disturbing performance limitations. ... It comes down to a single word -- hypoxia," Ward said at the time.

"The likelihood of the JSF displaying a flaw similar to the F-22's is high because the JSF is just as complex as the F-22, if not more so, and all that complexity tends to hide bugs and problems and flaws," he said.

-- Oriana Pawlyk can be reached at oriana.pawlyk@military.com. Follow her on Twitter at @Oriana0214.

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