Five "out of place" packages triggered an evacuation of 1,500 employees at NORAD for more than four hours on Thursday while the materials were scanned and removed for further inspection.
NORAD, or North American Aerospace Defense Command, is a joint operation between the United States and Canada that maintains a 24-hour-a-day, seven-day-a-week capability to detect potential threats to either nation.
Based out of Peterson Air Force Base in Colorado Springs, Colo., NORAD control at the time of the evacuation was already operating out of a backup facility.
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Peterson Air Force Base spokesman Jeff Bohn refused to provide further information about the "out of place" packages, saying only that tests ruled out chemical, biological and radiological agents.
Bohn said he was unaware of how the packages arrived and whether they were screened before being delivered.
In 2006, NORAD relocated its facilities to the Peterson Air Force Base from a site several miles away carved out of Cheyenne Mountain which was designed in the 1960s to withstand a nuclear attack by the Soviet Union.
According to NORAD, the move was done to save money and consolidate personnel, though the Cheyenne Mountain control room remains fully operational as a backup facility.
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At its current facility at Peterson Air Force Base, NORAD shares its headquarters with the U.S. Northern Command, which is responsible for defending U.S. territory from attack. Officials said essential Northern Command operations were not interrupted.
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