The Air Force is close to understanding what is causing some F-22 fighter pilots to get dizzy and disoriented when flying the most advanced U.S. warplane, and could resume normal operations later this year, a top general said on Thursday.

Air Force Major General Charles Lyon said he was “cautiously optimistic” that an ongoing investigation had identified the major factors causing symptoms of oxygen deprivation among some pilots of the F-22, built by Lockheed Martin.

Those factors include the size of hoses that deliver oxygen to the F-22 pilots, connectors between equipment, and a pressure vest that is meant to keep pilots safe at high altitude, especially when worn with other heavy clothing.

Lyon said the Air Force had nearly ruled out the idea that some contaminant was getting into the aircraft’s oxygen system, and in April it had stopped requiring pilots to use a chemical warfare filter after realizing it made it harder for them to breathe.

He said there was still a chance additional factors could come to light in the next month or so, and the Air Force would not rush to judgment.

The Pentagon last month announced new safety precautions for the most advanced, stealthy U.S. warplane, including limits on how far they can fly from airstrips.

The Air Force continues to investigate the issue, but this week told pilots to stop wearing a potentially faulty pressure vest that might be contributing to the issue during training flights and routine operations below altitudes of 44,000 feet.

The Air Force stopped flying the F-22 completely for five months last year, resuming flights in September. But concern about the F-22 flared again this year after two F-22 pilots told CBS’s “60 Minutes” program they had stopped flying the fighter jet due to worry about safety.

Two U.S. lawmakers on Thursday released new Air Force data showing that F-22 pilots were experiencing symptoms of oxygen deprivation, or hypoxia, at 10 times the rate seen on other U.S. warplanes.