In the predawn hours of Sept. 2, at the plant that stores the nation's stockpile of highly enriched uranium, guards wearing body armor and carrying loaded submachine guns were dispatched to intercept a group of men who had apparently set off an intrusion alarm. But the target group turned out to be a second team of guards, who were conducting a mock attack with laser-tag equipment.
The armed guards, a "shadow force" maintained in reserve during such drills, rushed through the dark, ready, people involved said, to shoot at a group whom they believed were intruders.
Such a deployment is virtually unheard of, security experts said, and had it led to a shooting, the incident could have destroyed the ability to hold such drills, a crucial tool in determining if the plant is adequately defended. The plant, called Y-12, is owned by the Department of Energy but is defended by a contractor, Wackenhut.
"For two minutes, it was mass confusion," said one of the guards on duty that night. "People asked several times, 'Is this a drill?' Nobody would clarify."
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He and another guard involved in the incident agreed to speak only on condition of anonymity, saying they had been threatened with firing if they spoke with outsiders about the incident......
http://nytimes.com/2004/12/21/national/21nuke.html