Systems designed to provide breathing air and pressurization for pilots of the T-45 Goshawk and F-18 Hornet are the prime suspects in a rash of oxygen deprivation incidents, the Navy said Thursday.

Pilots are at risk of breathing contaminated air from the on-board oxygen generation system used on the aircraft, while aging parts and inadequate testing of the fleet have caused failures in the cockpit environmental control system, according to a safety review by top Navy officials released Thursday.

Still, positively identifying the root cause and finding a solution to widespread reports of oxygen deprivation among pilots has proved elusive, the review found.

"We have to start with the sources of where we get the air, so either the oxygen system or through the cabin air system, which is filtered off the bleed air coming off of the engines," said Adm. Bill Moran, vice chief of naval operations. "When I say we have not found the cause … it may be more than one component or it may be more than one condition that clearly leads to a physiological event."

Moran said the Navy is working closely with Boeing, the prime contractor for both aircraft.

Both aircraft have seen spikes in reported incidents of oxygen deprivation that can cause sickness, hyperventilation and panic in pilots.

The T-45 trainer jets were grounded in April at bases in Mississippi, Florida and Texas following a boycott by pilots including Vice President Mike Pence's son. Congress has been concerned about F-18 oxygen systems and directed a review in December.

The Air Force is now experiencing its own reports of oxygen deprivation and grounded a squadron of its F-35 fighters at Luke Air Force Base in Arizona last week.

"The integration of the on-board oxygen generation system … in the T-45 and FA-18 is inadequate to consistently provide high quality breathing air," the Navy concluded in its review. "To varying degrees, neither aircraft is equipped to continuously provide clean, dry air to [the oxygen generation system] — a design specification for the device."

The problem allows contaminants to pollute the breathing air, which can cause oxygen deprivation.

Meanwhile, the system responsible for pressurizing the aircraft cockpits is a "complex aggregate of sub-components, all of which must function for the system to work as a whole," the Navy found.

"Aging parts, inadequate testing methodologies and numerous other factors are impacting fleet [environmental control system] reliability, inducing several instances of decompression sickness," according to the service.

Pilots in the T-45 reported 21 incidents so far this year — 10 in March alone — after a spike of 38 physiological episodes in 2016, the Navy review also found.

Reports more than doubled from 12 to 27 in the two preceding years.

Moran said the Navy has designed a new oxygen mask for the T-45 that is now in use and is installing water separator mechanisms that are already used on other high-performance jets in hopes it could solve the problems. That could be completed by early fall.

No student pilots are currently flying the T-45, which means 25 have been delayed each month since late March, the Navy said this week.

Meanwhile, pilots of the F/A-18 Hornet, along with the EA-18G Growler — the service lumped the data together — experienced 52 incidents so far this year after a major spike of 125 last year. It has been a dramatic rise since 2009, when there were just 16.

But the Navy review also cast doubt on those statistics, saying past instances might have been under-reported.

The increase of incidents beginning in 2010 was "likely more reflective of a change in aircrew awareness and reporting mechanisms than a sudden rise" in oxygen deprivation incidents, it said.