Navy student pilots have not trained in the T-45 Goshawk aircraft for more than two months due to a mysterious oxygen system problem that has puzzled the service, Vice Adm. Paul Grosklags, commander of Naval Air Systems Command, said on Tuesday.

"To date, since the beginning of April, end of March, we have not flown any training events with the students, the students have not flown at all," Grosklags told a Senate subcommittee.

Navy instructor pilots are flying the T-45, but they are not using the oxygen system and are remaining below 5,000 feet as the service runs exhaustive tests after numerous reports of hypoxia.

The lack of student flights is delaying the progression of 25 students per month, or a total of 75 Navy pilots by the end of this month, Grosklags said.

The hypoxia incidents, called physiological episodes by the Navy, caused a boycott by hundreds of instructor pilots, including Vice President Mike Pence's son, in April.

The service grounded the aircraft at the time and sent T-45s from facilities in Texas and Mississippi to Naval Air Systems Command for a full evaluation.

F/A-18 Hornet fighters have had similar problems, and the Air Force has grounded F-35 joint strike fighters at Luke Air Force Base in Arizona after some pilots reported incidents of oxygen deprivation.

Grosklags told the Senate that the Navy has since torn the T-45 aircraft completely apart, testing the chain of components from the jet's engine to the pilot's oxygen mask.

T-45s have also been fitted out with equipment to detect any potential causes during flight, but that also has been unsuccessful in identifying the cause, Grosklags said.

"We are not doing well on the diagnosis," he said. "It would be a lot easier if we could determine what the root cause is and go after that root cause. To date, we have not been able to identify a smoking gun."

The Navy is now working to install about a dozen alert measures that could detect problems before pilots and students are affected by oxygen system problems, which could allow flights to resume within weeks.

"If a person is sick, the most important step is a good diagnosis," Sen. Roger Wicker, R-Miss., said. "We are having a diagnosis problem with the physiological episodes."