The first thing I’d do is to do anything and everything to try and get some sort of a police report submitted that I can get a copy of. The next thing would be to search the IRS website for the keyword “fraud”, etc. and do the same thing with SSA...I’d then back this up with a call to the IRS and to the SSA...if you are persistent, you will eventually get a real person.
If you get clear of this, I’d invest in Lifelock for $9/month.
Be prepared, you may end up having to consult a lawyer specializing in identify fraud. Your state bar association should be able to give you referrals.
Good Luck....BTW...I’d get some new W-4s filed on your current income that cuts your tax bite to the bone so you don’t lose what you pay each month when you try to file next year.
Agree too; on the 'Life Lock' - our family has this. Perhaps; you could sign up now. . .(explain sit) but using your own SSN; they could track what is happening and be of some help; don't know. Might be faster than Gov tracking; but of course; the Gov must be part of solution as well.
Hope no one is yet; taking advantage of your medical benefits; much less your SS check.