Whoa. Are they worried about biological/chemical agents or is there another reason to do this that I'm too dense to figure out?
LIVE FEED AS IT IS HAPPENING
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Not being there, and having no information beyond what you have ... I would say that this would be a 'Standard Operationg Proceedure' in case of an attack on base for any reason. Rather safe, than sorry; right?
“Alarm on base, according to his Fort Hood source, signals all persons to shut down heating/air conditioning and take immediate cover.”
Many years ago I worked at a military facility that dealt with nuclear training programs. We had coded alarms which included fires and the series of beeps indicated where the fire was located. Then we had a lockdown alarm, to put away our classified documents, lock down the offices and prepare for further instructions. The third alarm was for criticality, which basically instructed us to drop everything and head for the bus lots for evacuation (although I suspect it meant really to just kiss your you-know-what bye-bye). At any rate, if authorities wanted to get everyone to stay put and lock up, thereby clearing the grounds for searches and possibly exposing runners, then the lockdown alarm would seem to be the best bet. And it would protect people from accidentally running into a perp. (Incidentally, for about a week we kept having a spurious crit alarm go off. Longest week of my life.)