Product: Credit card or prepaid card

To Whom It May concern : I would like to start a formal complaint against my credit card issuer- CITI VISA Signature Costco XXXX My credit card was lost/ stolen on or around XX/XX/XXXX. As a result fraudulent charges have been made on the card for XXXX XXXX I was contacted by the card company on XX/XX/XXXX asking if these charges are legitimate. I let them know immediately that these are fraudulent charges. The card service representative stated they will conduct investigation and let me know the outcome. They have never refunded the disputed money back on my card. They instead sent me a letter few weeks later that charges are not going to be reversed and they are legitimate and have to be paid in full or I will face penalties. The reason given was that the card chip was used at the time of purchase and therefore they believe it must have been made by me. I did sent them a certified letter via fax/ email and USPS where my wife and I confirmed in writing that we have not made the charges on the card. The credit card security department first denied they have received the letter. Later they said they have it but will need a police report. I did provide them with the police report and my driver license. I have personally contacted the stores where the charges were made. Unfortunately they could not provide video or any other info on who made the charges. One of the merchants said they don't even require a signature when the card is used. Its very unfortunate as this particular card even has my picture on it. I spent a lot of time and effort trying to solve this matter. I have logged all my calls with different representatives from XX/XX/XXXX until last call on XXXX more then 5 hours of call time over four months. On XX/XX/XXXX the representative assured me they have all they need and still working on the case and it may take 2 billing cycle and that they will notify me. Since I never received any notice from them and the charges were never removed from the account I reached out again on XX/XX/XXXX. The representative said that case was closed on XX/XX/XXXX and the charges will remain my responsibility. When asked for the reason of their decision the agent ( XXXX XXXX gave the same explanation- the card chip was used and therefore it must be the owner of the card that made the charge. When I asked them do they have any proof such as signatures or video on file showing me or my wife making the charge they said no. I have kept call log with names of people I spoke and time spent along with all letters, documents submitted. These are available upon request. 
My wife and I we suffered emotionally throughout this process and we feel that we are double victims- once our credit card was stolen and fraudulent charges were made and second we have been treated unfairly by the CITI VISA credit card services . They did not even had the courtesy to notify us they have completed their investigation. We are not even sure if they had done any investigation at all. They never refunded the money disputed even as a temporary credit to the account. 
We are determined to seek our consumer rights in the above matter. We are readily available to provide any further info if needed. 

Thank you for your consideration, Very Respectfully, XXXX and XXXX XXXX