Luke Air Force Base in Arizona, which canceled its F-35A Lightning II flights June 9 after five pilots experienced oxygen deprivation-like symptoms, will continue the grounding “until further notice,” the Air Force said June 13.

Keeping the flight suspension in place will give the Air Force more time to analyze the incidents and explore possible steps to prevent them in the future, said Capt. Mark Graff, an Air Force spokesman. 523a2574ad5b9-f-35a

“The safety of our airmen is paramount, and we will take as much time as necessary to ensure their safety,” Graff said.

The five incidents at the training base have occurred since May 2. In each case, the aircraft’s backup oxygen system worked properly, allowing the pilot to land safely, according to the Air Force.

The grounding applies only to Luke, which has 55 F-35As, the conventional-takeoff-and-landing version of the Lockheed Martin [LMT] three-variant fighter. The company has delivered more than 220 operational F-35s to the U.S. military and allies.

“Our team stands ready to help in any way possible to address this issue,” Lockheed Martin spokesman Michael Rein said.