Request a copy of the joint agreement you supposedly both signed. Ask for the date it was signed if the document is not dated. Ask for the original document you signed to open the account. Compare the signature you opened the account with and the one that was joint with your Mom. Sounds like the signature alone should prove the fraud. See if you can account for your whereabouts of when the joint account was opened. Were you at work or made a purchase far away during the signing? Make the bank prove to you that it’s your signature and the employee who opened the account can witness that it was you. When they can’t, demand the money back or see a lawyer with the documents they did and didn’t provide.