You can go to the FBI website and report this information ... they have a link for reporting tips. I'd advise you to share your concerns with them. I have done this and wouldn't hesitate to do it if the situation warrants. You'll feel much better if you do, and you have no way of knowing if perhaps there's an ongoing investigation your information could help. Do it, and then you'll know it's in the right hands and off your worry list.
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If one does report suspicions, does anything happen? Do they acknowledge?
What if one is in a capital city, and people with cameras appear to be innocently photographing 'architecture' or 'historical sites'?