The message (if using the Apple Messages app) includes the email address. Just track that down.
you can call the apple store and tell them and report it and give them the email address and they can look at the account and tell u!!! or u can call 911 and show texts and they can track down!!!!! trust me i know!!!!!!
Apple will do nothing. You can talk to the police, or maybe the FBI. Remember that the FBI recently go involved with email regard General Petraeus but I would not get my hopes up.
Turn off iMessage for a little while. They might see that they are not able to message you, and then give up.
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