What to do when Lexus dealership has damaged the bumper of my car that I brought in for key replacement?

I bought a lexus that needed a key replacement ( i didnt get a car insurance before i could start driving that car) . I dropped off the car at the Lexus dealership a month ago. They replaced the key, battery, and did some other work. When I was picking my car up three days ago from the dealership, i paid, signed the total, and sat in my car. Car was making a noise. When I got out to look at it, the bumper was messed up, one piece was broken, left light wasnt working. They claim that the towing truck (third party) did it when they dropped off, I m positive, that the damage wasnt there when I dropped it off. What do I do? I have a feeling they ve been driving that car for a month, before they called to tell me it was done. They wont let me see the videos from surveiliance cameras, or talk to the owner. The service was $1600, and now my bumper is crashed, and they won't take a responsibility for that.