One of the US ballistic nuclear missiles stored in an underground launch facility in Colorado was damaged in a technical crew “mishap” during maintenance, the US Air Force has revealed after concealing the “sensitive” incident for over 20 months.

The incident, which happened on May 17, 2014 was blamed on three airmen, however the human error did not pose a risk to public safety, the Air Force told the Associated Press. Since the accident, the damaged missile was removed from its silo.

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An investigation into the matter has been started by the Accident Investigation Board. However, it has not been made public as the summary offered to the AP said that the full report was classified on Nov. 9, 2015, by Gen. Robin Rand, commander of nuclear forces.

No further details were offered by the US Air Force because of the sensitivity of the matter. The launch site that stores the intercontinental ballistic missiles (ICBMs) is operated by the 320th Missile Squadron of the 90th Missile Wing in Colorado.

The only information offered was said that during a diagnostic test on the evening of May 16, 2014, a Minuteman 3 intercontinental ballistic missile “became non-operational.” It added that the next day, an unnamed “mishap crew” chief, failed to “correctly adhere to technical guidance” when performing the troubleshooting inspection, “subsequently damaging the missile.”

The Air Force team and their supervisor “lacked the necessary proficiency level” to understand the consequences of their actions, which resulted in some $1.8 million repairs costs, the summary added.

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The Minuteman missiles are kept in silos to protect from US nuclear deterrent against possible attacks. Each silo measures 80ft in depth and 12ft in diameter.

According to the Federation of American Scientists, that was founded in 1945 by researchers who worked on the Manhattan Project to develop the first atomic bombs, each missile site spreads over a surface area of up to three acres, with each silo connected to an underground launch control center through a system of hardened cables. A crew of two officers in the launch center provide 24/7 alert.