The head of the nuclear missile wing at a base in Montana resigned on Thursday and nine officers were removed from their jobs over a test-cheating scandal that involved 91 missile launch officers, the Air Force said.

Lieutenant General Stephen Wilson, head of the Air Force's Global Strike Command, said Colonel Robert Stanley, commander of the 341st Missile Wing at Malmstrom Air Force Base, had resigned on Thursday and would retire from the service.

The nine other officers, mainly colonels and lieutenant colonels, were removed from their positions of command at the Montana base that is home to a third of the nation's nearly 450 intercontinental ballistic missiles. They will be reassigned to staff jobs and face discipline ranging from reprimands to courts martial for failures of leadership.

Wilson said the root of the problem was the emphasis on perfection in the nuclear mission at the Montana base and throughout the missile force, which led to cheating on exams in an effort to achieve the sort of perfect scores perceived to required for advancement and promotion.

The exams were classroom tests to check staff knowledge of how to carry out the nuclear mission and security procedures.

"Leadership's focus on perfection led commanders to micro-manage their people. They sought to ensure that the zero defect standard was met by personally monitoring and directing daily operations, imposing unrelenting testing and inspections with the goal of eliminating all human error," Wilson told a Pentagon news conference.

He and Air Force Secretary Deborah James said the evaluation and assessment of missile launch officers would be radically overhauled in an effort to change the culture and behavior that has developed in the missile wing.

Nuclear critics say the problem is deeply rooted and has been going on for years, becoming increasingly acute since the end of the Cold War as the nuclear mission has increasingly come to be seen as a dead-end career that's relevance is in decline.