I agree with edpc's assessment above.
There is no "there" there.
Occam's Razor:
The reason they "aren't finding anything" is because there is next-to-nothing to find.
The guy left no manifesto, no explanation, not even a good-by note.
He told no one of his plans (unless the girlfriend is still stonewalling).
He amassed his arsenal quietly.
He moved it up to the room piecemeal and unobtrusively.
Again, a quiet, determined, and evil mind.