Collection agencies/validation

I have been writing letters to the credit bureaus to validate collection accounts that are not mine. I received responses that the accounts have been validated. I then sent letters to the collection agencies asking for proof of the debt signed contracts etc and received no response. I did receive on statement from a collection agency that was simply on their letterhead, my ss# and an address listed I had never heard of with an amount due. I then listed all these accounts in a letter to the bureaus with a small paragraph of the collection account, what I asked for (signed contract) and the fact the collection agency has not contacted me. I also listed the Fair debt collections practices act. This was done at the beginning of April. Anyone know of a better way to get their attention?