Attempts to collect debt not owed: Debt collection

I am currently applying for a mortgage, and a collection account from Sunrise recently appeared on my credit report. I have no knowledge of this account, do not recognize the alleged debt, and did not authorize or open any account connected to it. I sent Sunrise a written dispute by certified mail 10 days ago, but I have received no response. This account is fraudulent and does not belong to me. It is negatively affecting my credit and threatening my mortgage approval. I am requesting that Sunrise immediately investigate this matter, provide proof that the debt belongs to me, and remove the fraudulent collection account from all three credit reports as soon as possible.