Am I responsible for roommate's negligence?

Me and 2 other college roomates are in the process of having a lawsuit against us by our former landlord and her insurance company for damages caused by a pipe bursting during the winter break because they claim we left a window open and turned our heat off, which I am positive we did not and I am not even sure how they would prove we did. I contacted my roommate to ask what he was doing about the situation. He then told me that he was the one who turned off the heat in our living room, and that he has told that to our landlord's attorney in his answer to the complaint. Do they have a case against me? If I was not the negligent person AND he admitted to his negligence how could they hold me at fault?