WASHINGTON (AP) — Six Air Force officers who were in charge of caring for the infrastructure, fuel and logistics support for a North Dakota nuclear missile base were relieved of command due to a loss of confidence in their ability to carry out their responsibilities, the Air Force said.

The officers include two commanders and four subordinate officers at Minot Air Force Base, including 5th Mission Support Group commander Col. Gregory Mayer and 5th Logistics Readiness Squadron Maj. Jonathan Welch, a defense official said. The four subordinate officers were not identified.

The official said the dismissals were based on non-compliance with safety regulations for vehicles and equipment, and while the decision to relieve the officers of command was based on the results of one safety inspection, the units had not been compliant for some time. The official was not authorized to discuss details of the firings publicly and spoke on condition of anonymity.

The official said the dismissals reflect the tougher line the Air Force has taken on discipline within its nuclear ranks, which have weathered a series of safety concerns and controversies.

In 2007, a B-52 Stratofortress took off from Minot mistakenly loaded with six nuclear-armed AGM-129 cruise missiles and flew across the country to Barksdale Air Force Base in Louisiana. In 2014, a nuclear safety inspections cheating scandal at Malmstrom Air Force Base in Montana embroiled scores of missileers and officers, and in 2016, investigators busted an LSD drug ring at F.E. Warren Air Force Base in Wyoming. Minot, Malmstrom and F.E. Warren are home to a total of 450 silo-based Minuteman III intercontinental ballistic missiles.

In a statement about the firings, Maj. Gen. Andrew J. Gebara, commander of 8th Air Force, said the responsibilities for the nation’s warheads were a “no fail” mission.

“We have very deliberate and disciplined inspection protocols and we expect 100% compliance. It’s that important to us and anything below that threshold is unacceptable,” said Air Force Col. Brus Vidal, a spokesman for Global Strike Command, which is responsible for silo-based and bomber-launched nuclear weapons.