Best to call them on the phone. Look up your nearest FBI office in the GOVERNMENT section of your phone book. If it is after hours, you may not get through so try to get the regional number from your police. Regional offices are on 24/7; they will screen your call and if it is the least bit suggestive of needing investigation they will put you right through to an agent and will treat it seriously.
Here is a link to all the offices with phone numbers:
I recommend contacting them directly unless it is a total emergency, in which case call your local police.
THAT type of info should come immediately to the attention of the local FBI office.