It has been a long time since I outprocessed Fort Hood (back in 1990) but if it was like the processing center at Fort Lewis it is essentially a gymnasium converted into a large processing center where people go through stations that cover things like their wills, their shots, ensuring that all their equipment is taken care of, that family care plans are put together, etc.
Someone coming through the front door could open fire and probably could shoot quite a few people in the confusion, especially if he knows where the exits are and knows how the traffic will flow. I suspect he had been there several times in his role as psychiatrist since there is a psychological eval section as part of the demobilization process for soldiers returning from deployment.
During the busiest part of the day I remember seeing over 500-750 troopers in the processing center at Ft. Lewis. I suspect the one at Fort Hood was even larger since there is a huge combat arms presence there.
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