I received an offer from the FBI in 2003 for something I did in 2001. I was similarly distrustful even though I had already been caught and punished. It was the most surreal and frankly unnerving experience.
I think a combination of it being pre 9/11, me being 15, and expensive lawyers helped things along. I was arrested in February of 2001 and it was done by my birthday in September.