NORFOLK, Va. — The U.S. Navy on Saturday said it will allow a fleet of its training jets to fly again under modified conditions while it determines what’s causing a lack of oxygen in some cockpits.

Vice Adm. Mike Shoemaker said in a statement that its nearly 200 T-45C aircraft will resume flights as early as Monday after being grounded for more than a week.

Its pilots had become increasingly concerned late last month after seeing a spike in incidents in which some personnel weren’t getting enough oxygen. The concerned pilots had declined to fly on more than 90 flights.

Instructors and students will now wear modified masks in the two-seat trainers. They will also fly below 10,000 feet to avoid use of on-board oxygen generating systems.

The planes train future Navy and Marine fighter pilots. Shoemaker said students will be able to complete 75 percent of their training flights as teams of experts, including people from NASA, “identify the root cause of the problem.”

Two T-45s are now at Naval Air Station Patuxent River in Maryland where the teams are taking them apart to figure out what’s gone wrong.

“This will remain our top safety priority until we fully understand all causal factors and have identified a solution that will further reduce the risks to our aircrew,” Shoemaker said.

The Navy operates the training planes at three naval air stations in the Southern U.S. They are NAS Meridian in Mississippi, NAS Kingsville in Texas and NAS Pensacola in Florida.

Since 2015, the number of “physiological episodes” has steadily increased among personnel who fly in the plane.

Symptoms of low oxygen can range from tingling fingers to cloudy judgment and even passing out, although Navy officials said conditions in the trainer jets haven’t been very severe.

Cmdr. Jeanette Groeneveld, a Navy spokeswoman, told The Associated Press on Monday that nine people out of more than 100 affected since 2012 have been required to wear oxygen masks after a flight.

The T-45C was built by Boeing based on a British design. It has been operational since 1991. Production stopped in 2009, according to Groeneveld.

Each plane cost $17.2 million to produce, according to the Navy’s website.