You have sent a dispute letter to the credit bureau requesting them to delete negative information on your credit report because it is "not yours." If, however, the credit bureau is able to investigate the account within 30 days and confirms that the negative account is correct, you should try an alternative method in getting the negative account deleted.
The truth here is that not many people know about this method. What you will do, should the credit bureau report to you that the negative account is correct, is dispute the account with the original creditor or information furnisher. Write a similar letter you sent to the credit bureau to the creditor or information furnisher, requesting them to investigate the accuracy of the negative account in your report.
Let them know that you wrote a letter to the credit bureau but they reported that the account was accurate. Tell them that you know that the account is not yours and as a result it should not be recorded in your credit report and as a result of this, you want them to begin an investigation into the validity of the account.
Notify them that you have the right to make this request under the laws of the Fair Credit Reporting Act, FCRA. You should also include in your letter to the information furnisher that they should inform the credit bureau with the negative information / account to delete the negative account from your account if they are not able to confirm its accuracy upon conclusion of their investigation.
Add to your letter that the law gives you the right to take legal actions if they do not resolve this issue as prescribed by the law.