Question for Snafu

I don't know if you are still around, but I have a question for you since you have experience as an ATC.

What can you tell me (if you don't mind) about security clearance? I mean what is your take on the possibility that ATCs were made to keep quiet about 9/11 if they saw something suspicious? What is the procedure for that kind of thing? For example weren't there times when the military would fly experimental aircraft that would be seen by ATCs and thus they had to keep quiet for reasons of national security?

I guess I really don't know how to ask the question I am trying to ask. Basically I am just trying to get an understanding of how things such as security factor in to ATCs. I am sure it varies between military and civilians.

Thanks for any insight.