The shooter knew where the TARGETs were. He did not know the AI and student were in the house.
There are a dozen simple things that could have prevented this incident. At the very least propper target ID. (See point number one in my original post.)
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We're obviously not reading the same thing.
Shooter setup the targets. Check.
Class runs drill. Check
Shooter exits. Check
AI + 1 stays. Check
Shooter re-enters and engages "targets" in the dark. Fail here
Shooter ND shoots AI standing in front of target. Check
Like Jerry said, headcount fail.