The 71st Flying Training Wing at Vance Air Force Base, Oklahoma, resumed flying T-6A Texan training flights on Tuesday.

But the cause of the hypoxia-like symptoms that led Vance officials to ground the Texan fleet Nov. 15 has not been found.

In a release, Vance spokeswoman Terri Schaefer said that after a two-week investigation, aviation, medical, functional and industry experts could find no specific cause for the hypoxia events that affected four instructor pilots and one student pilot in four different flights since Nov. 1.

“The cross-functional cooperation between agencies spanning the Department of Defense and industry has been outstanding,” Col. Darrell Judy, commander of the 71st, said in the release. “We are actively using lessons learned across the aeronautics enterprise to determine the cause of these events, and are confident implementing mitigation techniques will enable us to return to our primary mission of training the world’s finest aviators.”

Schaefer said they eliminated some possible causes, such as problems with maintenance and aircrew flight equipment procedures.

“As technical and human performance data continues to be gathered, the Vance team will temporarily apply local procedures to mitigate risk to flight operations and aircrew,” Schaefer said.

Hypoxia is a physiological symptom caused by too little oxygen in the body.