WASHINGTON — Nearly half of the officers responsible for maintaining and operating nuclear-armed missiles at a Montana base have been implicated in a widening cheating investigation, a sign of deep cultural and command problems in the nuclear force, the leader of the Air Force said Thursday.

Air Force Secretary Deborah Lee James said at a Pentagon news conference that 92 of 190 launch officers at Malmstrom Air Force Base had been suspended because of the investigation into the sharing of answers on a proficiency test last year.

For the first time since the Air Force disclosed the cheating scandal this month, officials acknowledged that the cheating stemmed from a climate of fear created by commanders, who decided which officers on launch crews would be promoted based on whether they scored perfectly on monthly tests.

“I believe that we do have systemic problems,” James said. “The need for perfection has created a climate of undue stress and fear.”

The number of implicated officers has nearly tripled since Jan. 15, when the Air Force announced that 16 had shared text messages with answers to a monthly missile proficiency test and that 17 others were aware of the suspected cheating but took no action.

Air Force officials continued to say they had no firm evidence that cheating went beyond the single test last year or that it had occurred at the two other Minuteman III missile bases, one in North Dakota and another in Wyoming. They acknowledge, however, that a climate of fear exists at all three installations.

The 550 launch officers at the three bases take three written tests each month on missile safety, handling of launch codes and classified war plans. They also complete a monthly test in a simulator and an annual inspection, along with periodic unannounced inspections.

Eight times a month, a two-man crew completes a 24-hour shift in an underground launch center.

The cheating investigation is focusing on a “core group” of about 40 officers at Malmstrom who are believed to have been most involved in sharing test answers, said Lt. Gen. Stephen Wilson, head of the Air Force Global Strike Command, which oversees missiles and long-range bombers that carry nuclear weapons.

The rest were aware of the cheating but may not have used the answers, he said.

With all 92 officers under investigation suspended from launch duty, the remaining crews at Malmstrom have been required to work more shifts, and senior officers have been moved back to launch duty to keep the base’s 150 missiles operational, Wilson said.

Wilson said he had a “force improvement program” to fix what were described as systemic problems that the intercontinental ballistic missile force faces. He said commanders would be disciplined if they were found to have contributed to those problems.

“We’re going to take this wherever it goes,” he said.

Some former launch officers say that cheating in various forms on the tests has been common for decades, though they say the pressure to achieve perfect scores has increased in recent years — ironically, as the importance of nuclear weapons to U.S. national security has declined with the end of the Cold War.

After a 2007 incident in which nuclear weapons were mishandled at Minot Air Force Base in North Dakota, the Air Force created Global Strike Command to reinforce the need for rigorous attention to the secure and reliable handling of the weapons.

The result was even more pressure on launch crews and more tests, said Bruce Blair, a former Minuteman launch officer and co-founder of Global Zero, an organization that advocates worldwide elimination of nuclear weapons.

“Suddenly there were nothing but inspections going on constantly,” he said. “This new organization (Global Strike Command) needed a mission, and it meant that testing took on a life of its own.”

AFP Photo/Spencer Platt