MSPeconomist

Keep notes of any conversations with DL, including time, date, and name/location of the employee. I assume that you're talking to something like a fraud protection unit as well as the elite line or reservations/general FF people. Try to get the full name and phone number of the person responsible for your case. You might want to also file a report of the missing miles by email and even snail mail, just as you would do for any fraudulent credit card charges. (I think thieves have merchandise sent to drop addresses so it might not be so easy to catch the perpetrator, but you might want to file or make sure that DL files a police report in the appropriate jurisdiction. Consider also informing your homeowner's or renter's insurance carrier promptly if it looks like this might be a covered loss, after the deductible, on your policy.)

I haven't heard of this with DL, but there have been some threads in other airline fora on FT about miles being stolen from accounts, IIRC AA and UA. Reading those reports could be useful and suggest how such situations are generally handled in the industry.