My husband had a similar thing only as a "personal debt" a few years ago. it was a supposedly "legitimate" law-firm that had "bought" bad debt and was trying to collect. My husband and settled and paid the debt years ago, but had lost the paperwork. Instead of US trying to come up with the proof, we called them out on it. they scheduled a court date for settlement, and because of the laws in our area, they HAD to file in the county we lived in to collect the debt. We sent formal notorized letters to both the law firm and filed with the court case requesting that hard proof of the debt purchase, purchase price, and date be submitted to the court before we would pay. we should up for court, and magically, that morning they had faxed in papers closing the case.
so, long story short, you, as the defender don't have to prove your innocence until THEY prove your guilt. Don't fill out that letter, make THEM prove that you are guilty. And it should all disappear, because they can't. They scare you into thinking you have to prove you're innocent, and a lot of people forget that in our court system you are innocent until PROVEN guilty. the initial burden of proof is not yours to present, it is theirs.