Fraud Investigation

I received a letter stating I was a suspect in a fraud investigation that occurred in the department I was working at. I`m the only girl who works there, so when the names was given, it was assumed that I was responsible for the fraudulent charges since they were made on an online shopping center for girls. I never even heard of the site. Calling the CitiBank Sears fraud services today yielded nothing. The customer support stated that there was nothing linking me to the actual fraud, just the fact that my supervisor gave my name and they don`t know what else to tell me. I called my supervisor to inquire about the report and she told that she was told to give out three names and she gave mine as well- while the investigation team said she only gave mine. At this point, I have not signed the Agreement form because I am not going to pay for anything that I did not do, even though the price is only about $300. Now that the security is unable to help, what do I do?