Just keep in mind that anything you give them can and likely will be used in a lawsuit against you for negligence and/or incompetence. If you have public-facing documents that explain your services then point them to those boilerplate resources. The bank can afford a lot of expert witnesses to explain to a judge where and how you went wrong.
Did they call the FBI? I'm serious about that. Do they use online banking? If they do I thinks there's little chance of them getting the money. The only thing protected in banks by the Feds are deposits. If they didn't use online banking then it's on the bank. Obviously they'll try to pass the blame but in the end it's their problem.
The very reason that one should offer not a single bit of information voluntarily. @Metanis is right that the bank will have a crack legal department that will make it their mission to try to pin this on someone else, and potentially make them cough up the cash.