Someone has bragged about desiring to KILL members of your family...and you do not want investigators to check if he is just an idiot or an idiot who HAS those exact intentions.
He cannot be arrested because he mentioned it. You hope someone can get close enough to him to find out what his “real” intentions are and when the evidence is gathered ARREST HIM..
DO NOT OBSERVE UNTIL HE HAS COMPLETED HIS MISSION..Destroy him before he can..
I am very happy to see that the FBI, CIA, NSA are doing their jobs to save Citizens lives, whatever the process requires (just do not tell wife about my other love interests)..