I read that the complex he lived in housed students from that University. Since classes ended in May, the roommates probably returned home for the summer or had graduated and moved out. If they moved out then the shooter had the place all to himself, to do freely anything he chose.
Next question is, since he dropped out of school in May, what was he still doing in that complex in July? Why wasn’t he evicted?
I don’t understand why the room mates have not been found or why he was still in the apt.