The 19th Air Force’s fleet of T-6 trainers were grounded after 13 unexplained events, such as hypoxia, in a single week, Air Education and Training Command said Thursday.

The bulk of the unexplained physiological events ― 10 of them ― occurred at Columbus Air Force Base in Mississippi, said AETC spokeswoman 1st Lt. Geneva Croxton in an email. Two other events occurred at Vance Air Force Base in Oklahoma, and another happened at Sheppard Air Force Base in Texas.

Croxton said that the unexplained events included conditions such as hypoxia, or too little oxygen in the body; hypocapnia, or too little carbon dioxide in the blood; and hypercapnia, or too much carbon dioxide in the blood.

“Each event is being investigated, and it is too early to speculate about potential causes,” Croxton said.

AETC announced the “operational pause” Wednesday evening, and said it was ordered by Maj. Gen. Patrick Doherty, commander of the 19th Air Force, to give the Air Force a chance to study the root cause of these problems, listen to and educate air crew, find solutions, and put them into place. The grounding took effect Thursday.

The 19th Air Force trains more than 30,000 U.S. and allied students annually in numerous air power specialties. Here, T-6 Texan pilots from Laughlin Air Force Base, Texas, train in 2004. (Master Sgt. Jeffrey Allen/Air Force)
The 19th Air Force trains more than 30,000 U.S. and allied students annually in numerous air power specialties. Here, T-6 Texan pilots from Laughlin Air Force Base, Texas, train in 2004. (Master Sgt. Jeffrey Allen/Air Force)

Earlier on Wednesday, AETC confirmed that the 19th had suspended all solo flights in T-6s due to concerns about events like hypoxia.