You did the right thing but the bank's reaction was appalling. I would have done what you did.

Write, or ask to speak to someone further up the tree. Or ask to speak to the chief internal auditor - there's obviously some control failure when this happens and people are trying to cover up.

I had a problem with a company in different circumstances and wrote to the chairman of the board not expecting a response but I got a personal reply and some credit vouchers.

The brown paper bag in the alley hmmmm? I'd still take it to the cops.