if it were me...
I'd get a lawyer to do some work to see if he is really doing it. Once he is under suspicion usually one of two things happen:
His department grabs him by the balls and forces him to stop or lose his job and possibly go to jail....
OR his department hides what he is doing and wont admit to anything.
Either way I would get in touch with a lawyer and the ACLU. Your right to privacy is possibly being broken.
Most police departments today can't afford any problems with their people, since they are under enough fire without having a loose cannon who is creating problems for them.
Long story short, if you think he is stalking you, GET A LAWYER, dont delete emails, dont delete anything that may incriminate your ex. If your serious about this then you need to act fast, like now!