How many have you thought to consider this? If you served in the military, and had any kind of security clearance, the the FBI investigated you, and have a file.
When I was in the Air Force, I was required to have a top secret clearance during my overseas stint. Got a letter from my dad wanting to know why the FBI was investigating me. We ended up having a good laugh about it after I got back home.
Now, from what I understand, old J. Edgar wanted a file on everybody in the U.S.A.
For what it's worth, Buffalo Springfield, "Paranoia strikes deep. Into your life it will creep. Starts out when you're always afraid. Step out of line, the man come and take you away."
There may be more truth to that than you may think.
Paul B.