The careers of four Air Force officers who held the keys and codes to launching nuclear missiles are likely over after they were severely reprimanded for leaving blast doors open on their underground launch capsules, Air Force officials told NBC News on Tuesday.

The two incidents occurred earlier this year at Minot Air Force Base in North Dakota and Malmstrom Air Force Base near Great Falls, Mont., the officials said. The officers involved weren't identified in the incidents, which were first reported Tuesday by The Associated Press. Security wasn't breached in the incidents, The AP reported.

But while the violations weren't criminal, the reprimands are considered "career-ending," Air Force officials told NBC News.

These are merely the most recent in a long string of security and safety violations involving the Air Force's stewardship of nuclear missiles, especially at the Minot base — one of the most critical in the U.S. nuclear structure.

In May, NBC News reported that 17 officers were suspended and disqualified from controlling nuclear missiles at Minot after a safety inspection earned a grade of "D."

In August 2007, six cruise missiles loaded with W80-1 nuclear warheads were flown from Minot to Barksdale Air Force Base in Louisiana before authorities discovered that the warheads hadn't been removed for safety, as required.

In a review of the incident in February 2008 (.pdf), the Defense Advisory Board blamed "process and systemic problems" at Minot that had "developed over more than a decade and have the potential for much more serious consequences."

Just five months later, three Air Force officers fell asleep at the controls of a component that contained old launch codes for nuclear ICBMs at Minot. They were immediately barred from working with classified and nuclear materials and were later discharged from the service.

In the newest incidents, the two-man launch crews at Minot and Malmstrom violated regulations that require heavy steel blast doors to be closed and locked when one of the crew members is sleeping.

Even though the tightly packed and cramped launch capsules are deep underground — and guarded by heavy security on the surface — the rule is strictly enforced. The blast doors could be a way for someone to gain access to command posts for nuclear sites housing 10 Minuteman-3 missiles, Air Force officials said.

The AP reported that in both incidents, the concrete and steel doors were deliberately left open while one of two crew members inside slept. One of the officers lied about a violation but later admitted to it, according to The AP.

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