What's happening with the investigation or whatever they're calling it?

Don't do anything rash. Set some arbitrary cooling down period before you go nuclear on your career. Not that I think you would ruin your career by resigning, but life is certainly easier when you're operating from a position where you have a job and you're not needing a job like right now. I know they've pissed you off royally, and they deserve your ire, but don't shoot yourself in the foot. Set a timer for yourself before you hit send on that letter.