Washington (CNN) -- The U.S. Air Force has grounded its entire F-22 Raptor fleet amidst continuing concern over how the fighter jet provides oxygen to the pilot, according Capt. Jennifer Faerrau, a spokesperson for the Air Force's Air Combat Command.

The stand-down order was made Tuesday but is just now becoming public. It's unclear how long the stand down will last.

There are about 165 Raptors in the fleet. Since January, the aircraft already had been kept at altitudes under 25,000 feet during an ongoing investigation into a November crash. Flying above that altitude could cause a pilot to black out from lack of oxygen and lose control.

The stand-down affects all routine training and other missions, Faerrau said.

The Air Force is looking at all the systems on this new-generation fighter jet, but one area of focus is the On-Board Oxygen Generating System (OBOGS), according to Ferrau.

The order came from Gen. William Fraser, commander of Air Combat Command, which is based at Langley Air Force Base in Virginia.