How to confirm if an alleged debt claim is correct.

NCO Financial sent me a vague document requesting payment of $6k for a debt they had purchased. I sent a letter requesting verification of debt and any legal documents pertaining to sale of such debt. I received no response, but weeks later received another request for payment. I resubmitted my request. I finally received a response advising they want to help me resolve the issue, but I need to provide them with my SSN and a copy of the original paperwork that was sent to me regarding the debt. What is the best way to respond to this request?

My initial response is to ignore it...if they can't find out why they sent me the demand for payment to begin with I shouldn't have to prove what was sent to me. Also..I would never give me SSN out, especially to someone on a fishing expedition. I considered responding in writing, but am leery to do so as the verbiage may cause problems down the line. Any suggestions?