WASHINGTON (AP) — Security guards at an Air Force base responsible for protecting strategic nuclear missiles in three states are under investigation for alleged marijuana use, Air Force officials said Monday. They are part of the same security force whose members were caught using the hallucinogen LSD four years ago.

Officials declined to disclose the number under investigation or provide other details, but they said Gen. Tim Ray, the top general in charge of Air Force nuclear weapons, flew to F.E. Warren Air Force Base in Cheyenne, Wyoming, to hold a “no-notice” meeting Monday to address the reported misbehavior by members of the 90th Security Forces Group.

Those under investigation have been removed from their duties until the probe is completed, the Air Force said. The 90th Security Forces Group is responsible for security at F.E. Warren as well as for the network of nuclear-armed Minuteman 3 intercontinental ballistic missiles — a key segment of the U.S. nuclear force — in underground launch silos in western Nebraska, southeastern Wyoming and northeastern Colorado.

This is the same F.E. Warren security group in which investigators uncovered and cracked a drug ring in 2016. Records obtained by The Associated Press in 2018 showed that airmen had bought, distributed and used LSD and other mild-altering drugs, as well as marijuana. That ring operated undetected for months; 14 airmen eventually were disciplined, of which six were convicted of LSD use or distribution.

The Air Force also operates Minuteman 3 missile bases in Montana and North Dakota. Those two bases, plus F.E. Warren in Wyoming, have been under scrutiny in recent years for numerous episodes of misconduct, including a cheating scandal and training failures. The Air Force has taken steps to address those problems, including investing more in training and equipment and launching a campaign to publicly emphasize the importance of the work done by this relatively obscure nuclear force.

Marijuana may be consumed legally in some states, but not in any branch of the U.S. military.

In a statement Monday, the Air Force Global Strike Command quoted Ray as saying “the majority of our airmen are exceptional and have made significant gains in ensuring excellence and adhering to exacting standards.” He added: “But we will not give up one inch of this hard-earned ground. When any of us see those not living up to our high standards, we will hold them accountable using all of the disciplinary tools available under the military justice system.”

The statement said no details would be provided until the investigation is completed “in order to uphold investigative integrity.”