After more than two months, the Navy still has no idea what is causing serious problems with the oxygen systems in its training aircraft and fighters.

“We’re not doing well on the diagnosis,” Vice Adm. Paul A. Grosklags, commander of Naval Air Systems, told lawmakers on Tuesday. “To date, we have been unable to find any smoking gun.”

In April, the Navy temporarily grounded all of its T-45 training jets after dozens of instructor pilots refused to fly, citing a spike in pilots suffering from dangerous symptoms caused by a lack of oxygen or contaminants in the oxygen system.

Navy Times first reported in May 2016 that Navy and Marine Corps F/A-18 Hornets and Super Hornets and EA-18G Growlers also experience oxygen system failures with alarming regularity. Worse: The potentially catastrophic failures have been becoming more frequent.

Most of the problems in the T-45s involve breathing gas, while the F/A-18s tend to have problems with cockpit pressurization, Grosklags told the Senate Armed Services Seapower Subcommittee on Tuesday.

The Navy has literally torn T-45s apart at Naval Air Station Patuxent River, Maryland, where investigators have tested every single component in the aircraft, yet the root cause for the problem remains elusive, he said.

“To date, we have not been able to discover a toxin or a contaminant in the breathing gas, despite our testing,” Grosklags said.

Since the end of March, students have not flown in T-45 trainers, Grosklags said. Each month the flights are suspended delays 25 students from graduating. After doing more testing, the Navy will consider resume student flights in “a matter of weeks instead of months,” he said.

Instructor pilots are not using the T-45’s oxygen system and they are restricted to flying under 5,000 feet and pulling less than two Gs, he said.

Grosklags was not given an opportunity at Tuesday’s hearing to address what the Navy is doing to remedy the oxygen issues with Navy and Marine Corps Hornets and Super Hornets.

Meanwhile, the Air Force’s variant of the F-35 Joint Strike Fighter is also experiencing problems with its oxygen systems. Flight operations at Luke Air Force Base, Arizona, have been suspended after five pilots from the 56th Fighter wing reported suffering symptoms from lack of oxygen.

In May 2012, F-22 Raptor pilots went public with their concerns about the aircraft’s oxygen system. Two months later, the Air Force determined the cause of the problem was a valve on the pilot’s Combat Edge life support vest, which improperly tightened, making it harder for the pilots to breathe.