How do I go about handing car loan fraud?

My mother took out a car loan in 2001. Originally, she took the loan as a 6 year car loan and the original interest rate was 8.25%. The car loan payment was being taken directly from her account as agreed by my mother and the bank. For about 4 years of the loan, the money was being deducted and at some point during the time, she realized that she had an outstanding balance left on the car loan. When she went in to the bank to question this, she received a lot of run around from the bank officers and finally she had to have a meeting. She pressured them to investigate what was going on with her account. She learned that it was still unclear how this error occurred, but they had been charging her the wrong interest rate(12.5% instead of 8.25%) for over 3 years of the loan and when the loan officers investigated this, they still couldn't tell my mother who signed the document for an increase in the interest rate. Also, she learned that instead of the payment going towards the princ