In January I rented a car through Enterprise for 9 days. Near the end of the rental I noticed a tire was getting low when I got on the road, because the wheel was pulling. I pulled into a gas station and attempted to fill the tire, but the valve was broken and would not accept any air. I had to change the tire to the spare and drove it less than a mile to the nearest Enterprise location. They took the car and gave me another. One of the employees did the usual body check before I got the new vehicle.

A month later I got a phone call from that location saying that they finally got around to fixing the tire and when they raised the car to change the tire, they noticed "massive undercarriage damage." The guy asked me if the tire blew out when I was driving fast, or if I had noticed any undercarriage damage when I rented the car. I told him the tire never even went totally flat, it was just low and unfillable, and that they should know that since they have the tire. Also, I am pretty sure you are not asked to inspect the underside of the car upon rental, so no I didn't notice any prior damage.
He kept trying to push me into saying that I may have caused this damage somehow but ultimately I told him there is no way I am accepting responsibility and that it was clearly a preexisting issue. He told me they would be filing a claim. I called my insurance, explained all this and the lady would not give me advise, but did tell me it sounded like they do not have a case and calling me a month after rental is suspicious. She said it would neither hurt or help me to file a claim, but I could just do nothing and wait to see if Enterprise follows up.

I got this letter in the mail earlier this week:

"Dear Sir/Madam:

Thank you for your recent rental. Our Damage Recover Unity has received notification of damage or loss to the vehicle you rented.

If you have not already done so, please contact your insurance company and/or credit card company to report this claim. Failure to report in a timely manner may void any coverage you may have. Once reported, please contact us to provide your claim information.

If you do not have insurance or wish to pay this claim yourself, we will send you a bill along with supporting documents. If you have any questions regarding your responsibility, please contact our office."

They included a form to be filled out and returned to them with my insurance claim information.

At this point, what is my next step? I have Googled this question and I have seen many responses. Some do nothing, some say to write a letter asking for all supporting documentation and a strong denial of responsibility. Others say to just file a claim with your insurance and let them handle it. My case does seem a little different than others I have read though, so I would love another opinion on my next step.