On June 28, a young U.S. Navy officer flying in a two-seater electronic warfare jet in the skies over Washington State suddenly felt a tightness in his chest and tingling in his extremities. He instantly recognized his symptoms as signs of hypoxia, or oxygen deprivation.

The jet, an EA-18G Growler from a training squadron out of Naval Air Station Whidbey Island, diverted to a local airport, and made an “uneventful” landing, according to Navy spokesman Cmdr. Scot Cregan. The crew member, an electronic warfare officer in training, was transported to a hospital for medical treatment.

Both the pilot and the trainee officer survived, but the incident, the latest in an alarming string of similar episodes, could have been deadly.

Across the U.S. military fleets, pilots and aircrew are experiencing a dramatic surge in so-called physiological episodes, which leave aviators disoriented and shaken. At worst, these unexplained incidents can be fatal — the Navy has linked four F/A-18 fighter pilot deaths over a span of 10 years to the events.

The continuing mystery over the pilots’ sickness is part of a deeper concern about the military’s aviation readiness, as the rate of fatal aircraft crashes recently reached a six-year high. It also raises larger questions over the ability of the world’s largest and best-funded military to resolve a basic problem that appears primarily limited to the United States.

The Navy considers the physiological episode problem its “number one aviation safety priority.” From 2009 to 2016, the rate of such events increased almost eightfold in the F/A-18 and EA-18G — a version of the two-seater F/A-18 — fleets, from 16 to 125 incidents. In the Navy’s T-45 training fleet, the spike is even more significant: In 2009, the Navy reported just one such incident, but in 2016, the number was 38.

Most recently, two aviators endured a harrowing landing after the temperature inside their Growler cockpit suddenly plunged to as low as -30 degrees. A mist formed in the cockpit, covering the instruments and windows with ice and rendering the pilots almost completely blind. The aircrew had to turn on the emergency oxygen supply.

The crew and ground-based controllers managed to work to land the aircraft safely. But both the pilot and the electronic warfare officer suffered “severe blistering and burns on hands” due to frostbite. The Navy believes the incident was caused by a failure of the environmental control system, a series of pipes and valves that regulates airflow to the air conditioning and oxygen systems.

The Air Force has a similar oxygen problem. In 2010, Capt. Jeff Haney died when an engine bleed-air malfunction caused the control system on his F-22 stealth fighter to shut off oxygen flow to his mask. Since then, the episodes have continued in almost every aircraft type, including the A-10 attack jet, the T-6 trainer, and the new F-35 fighter.

The most recent incidents have not yet been directly linked to fatalities. But in a sign that the military recognizes the severity of the problem, in the past year both the Navy and Air Force have grounded fleets in response to these events: the Navy’s T-45s in April 2017, the Air Force F-35s at Luke Air Force Base in June 2017, part of the Air Force’s A-10 fleet in November 2017, and the T-6s in November 2017 and again this February.

Neither service has identified a single point of failure or a solution to these episodes despite years of investment — a fact that has not gone unnoticed by prominent lawmakers.

“What’s occurring in the Navy is absolutely unacceptable,” said Rep. Mike Turner (R-Ohio), chairman of the Armed Services Subcommittee on Tactical Air and Land Forces, in 2017. “This is absolutely critical for our pilots.”

“I have no doubt the Navy is taking that issue seriously,” said Rep. Mac Thornberry (R-Texas), chairman of the House Armed Services Committee, during an event in Washington in 2017. But, “I don’t understand why we can’t figure out what’s causing the oxygen problem.”

In May, Rep. Adam Smith (D-Wash.), introduced legislation to create an independent National Commission on Military Aviation Safety in response to the surge in deadly crashes over the last year. Some, but not all, of these incidents were related to hypoxia.

Meanwhile, NASA has waded into the fray. After completing a congressionally mandated review of the Navy’s investigation into the F/A-18 and EA-18G incidents, which faulted both the Navy and manufacturer Boeing, the agency is embarking on a new study of how pilots breathe while flying high-performance aircraft.

The services continue making incremental changes to the aircraft design, flight gear, and maintenance procedures in an effort to mitigate the risk to aircrew. In the T-45, at least, these modifications have reduced the number of incidents, according to Rear Adm. Sara Joyner, who until recently led the Navy’s physiological episode investigation. Rear Adm.-select Fredrick Luchtman currently leads the effort.

But other Navy and Air Force fleets continue to see physiological episodes at alarming rates.

“More work remains to be done, and this will remain our top safety priority until we fully understand, and have mitigated, all possible PE [physiological episode] causal factors,” said Rear Adm. Roy Kelley, commander of Naval Air Force Atlantic, in congressional testimony June 21.