I have a capital one card that is very old and I know it has gone to collections, but I don’t know which collection agency. I have contacted Capital One since they are still listed on my credit reports as the creditor, and they cannot find the account in their system. I have disputed and disputed, but they come back and say the debt remains, even after they sent me a letter stating that they cannot locate me in their system or tell me what collection agency may have the account now.

What should I do
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Dispute the collection. I guess the capital one is not going away. You have tried. I assume you used certified delivery mail with copies of any documentation you have or should have saying not in the file.
I would tell you to re open with capital one but they are bad. So I will not. They report your credit limit as being whatever you charge so you are always maxed out with them.