And yet his folks knew something was wrong.
Probably because they knew:
1. He was disturbed and potentially dangerous.
2. One of their rifles, an AR-15, was missing.
I will not be surprised to find out that their call to police regarding their missing son hours before the assassination attempt included the fact that he was armed with a rifle.
No one calls the police about their 20 year old son being missing for a few hours unless they know something horrible is about to happen. A 12 year old, maybe. A 20 year old man, no.