A US Air Force security team's botched response to a simulated assault on a nuclear missile silo has prompted a damning review followed by expanded training to deal with the nightmare scenario of a real attack.
An internal review of the exercise held last summer at Malmstrom Air Force Base in Montana said the security forces were unable to speedily regain control of the captured silo and called this a "critical deficiency".
A copy of the report was obtained through a Freedom of Information Act request.
The previously unreported blunder was the reason the 341st Missile Wing failed a broader safety and security inspection.
The partially censored report indicated that the security team was required to respond to the simulated capture of a Minuteman 3 nuclear missile silo by hostile forces. The exercise was termed an "Empty Quiver" scenario, in which a nuclear weapon is lost, stolen or seized.
Each of the Air Force's 450 Minuteman 3 silos contains an intercontinental ballistic missile armed with at least one nuclear warhead and ready for launch on short notice on orders from the president. The Air Force review examined why the security force showed an "inability to effectively respond to a recapture scenario".
It cited a failure to take "all lawful actions necessary to immediately regain control of nuclear weapons".
A section elaborating on what was meant by the phrase "failed to take all lawful actions" was removed from the document before its release last week.
The Air Force said this was withheld in accordance with Pentagon orders "prohibiting the unauthorised dissemination of unclassified information pertaining to security measures" for the protection of "special nuclear material".
The document provided no details on how the silo takeover was simulated.
The Air Force recognised the possibility of such an intrusion as more of a danger after the September 11, 2001, terror attacks.
The prize for terrorists or others who might seek to seize control of a missile would be the nuclear warhead attached to it, since it contains plutonium and other bomb materials.
The 341st Missile Wing at Malmstrom has also been beset with other problems in recent months, including an exam-cheating scandal that led its commander to resign in March.