How the F.B.I. Can Detain, Render and Threaten Without Risk
Advice from my father-in-law, who is a prominent attorney: "Never, ever talk to the FBI without a lawyer - even if you want to help them as a witness... because if they don't like the truth you're telling them, they can (and often do) say you lied to them which is a federal offense.  If you have your attorney, they at least know there is a credible witness present who is keeping track of who said what."