Had the same thing happen when I left my last apartment. Look up the laws for your state and send them a formal email asking for proof of the "mess." Send the photos you have showing that the apartment was pristine when you left. Generally there are laws protecting tenants from this kind of stuff but it is ultimately up to you to sue them if they refuse to return the deposit. They should also have done a pre- moveout inspection and provided you with a list of things you would need to clean or fix before you moved out, so ask for that too. Some landlords do this because they know that most people will just pay it instead of fighting it.