By Robert Burns, AP National Security Writer

WASHINGTON (AP) — Fourteen members of an Air Force unit responsible for guarding nuclear missiles in Wyoming, Colorado and Nebraska are under investigation for possible illegal drug activity, including cases involving cocaine use, defense officials said Friday.

The probe is a fresh blow to a nuclear missile corps that has been under intense scrutiny in recent years for a string of lapses in training and personal conduct, first revealed by The Associated Press. The Air Force has said repeatedly over the past year that it is making significant changes aimed at lifting morale and improving performance.

The investigation at F.E. Warren Air Force Base, home of the 90th Missile Wing, near Cheyenne, Wyoming, was announced by Gen. Robin Rand, the four-star commander of Air Force Global Strike Command. The command is responsible for the entire fleet of 450 Minuteman 3 nuclear missiles that stand in underground launch silos, one third of them operated by the 90th Missile Wing. The missile force is on alert 24 hours a day, year-round, requiring strict adherence to performance standards by the men and women who operate, maintain and protect them.

Rand, speaking by telephone from his headquarters at Barksdale Air Force Base in Louisiana, said the 14 airmen under investigation range in rank from Airman 1st Class to Senior Airman and are members of the security group at F.E. Warren that is responsible for securing the missile fields and convoys that move nuclear weapons.

Rand said the 14 are accused of off-duty drug "activity," which he refused to further define. He said the allegations were "credible."

"This is very important to me that we get to the bottom of this," Rand said, adding that he is confident the vast majority of airmen in the nuclear missile corps comply with Air Force standards of personal conduct. "We have a special trust with our nation, with our public, with the mission that we do in Air Force Global Strike Command."

The security unit at F.E. Warren, known as the 90th Security Forces Group, includes about 1,300 airmen, Rand said, of which nearly 1,000 are junior enlisted members of ranks similar to the 14 under investigation. They are commanded by Col. Christopher L. Corley.

The investigation was started after a member of the security forces alerted his superiors of his suspicion of drug activity by another airman, Rand said, adding that the commander of the 90th Missile Wing, Col. Stephen Kravitsky, informed him Tuesday that an investigation was underway. It's not clear when it began.

Rand said the 14 have been removed from duty while the Air Force Office of Special Investigations looks into the case. He declined to provide further details, including what drugs are allegedly involved, citing an active investigation. Two other defense officials said the drugs included cocaine; a third said the allegations include the possession, use or distribution of illegal drugs. The officials discussed details they were not authorized to release publicly and so spoke on condition of anonymity.

The allegations do not involve officers who control the Minuteman missiles, officials said.

Security forces at nuclear missile bases are entrusted to patrol the missile fields and respond to any security emergencies. Just last month, Deputy Defense Secretary Robert Work visited F.E. Warren and observed a demonstration by security forces of the techniques and equipment they would use to recapture a Minuteman 3 missile silo that had been taken over by intruders.

Two years ago, while then-Defense Secretary Chuck Hagel was visiting F.E. Warren, officials disclosed that a number of launch officers, known as missileers, were under investigation for drug use. That led to the discovery that dozens of missileers had been cheating on their proficiency tests at Malmstrom Air Force Base in Montana, which also operates Minuteman 3 missiles. The 2014 drug investigations led to the dismissal from the Air Force last year of three missileers at Malmstrom who pleaded guilty to illegal use, possession or distribution of ecstasy.

Shortly after he learned of those drug allegations Hagel ordered a broad investigation of problems inside the Air Force nuclear missile corps, which had been extensively documented by The Associated Press starting in May 2013. At the time, he said, "Personnel failures within this force threaten to jeopardize the trust the American people have placed in us to keep our nuclear weapons safe and secure."

The Hagel-ordered review led to numerous changes, including providing billions more in resources to the nuclear missile corps and elevating the rank of the commander of Global Strike Command, which is responsible for the Minuteman 3 force, from three-star to four-star. Rand is the first four-star to hold the job.

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