What do I do if I am a victim of identity theft and have had money taken from Washington Mutual account?

There have been thousands of dollars missing from my account. I was not aware of this until I received a letter from a police investigator. I looked at my bank statements and saw there was a gap in the check numbers. I went to local Washington Mutual to get the returned checks and the writing was clearly not mine, the signature was clearly not in my name. The checks looked to be homemade with my account number at the bottom. Washington Mutual says there is nothing they can do and refuses to return my money. What do I do? Can I send a complaint to shut this bank down? I went online and have found that other people have had problems with this bank as well. How do I get my money back? I of course have canceled my account because initially WM said they would take care of it but there continued to be more unauthorized withdrawals from these fraudulent checks.