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Did you report your boss's activities to the FBI? Or is their visit to you just an investigation into his activities. If the latter, it's possible that if you speak to the FBI you could possibly incriminate yourself, even though you may not be under investigation. You should not put yourself into a situation like that unless you go to see the FBI with a criminal lawyer.
Unfortunately, sometimes there's only one right answer. And this one of those times where there's just no debate about it.
You have a 5th Amendment right against self incrimination and there is no criminal defense lawyer worth his license to practice who will tell you anything else.
If there's more you wish to know, you'll have to tell me what else you expected to learn about and I'll be happy to add to this answer.