You can ask your attorney to demand the full chain of assignments with copies of notifications to the mortgagee.
If you write the letter yourself, copy your state attorney general and the FDIC. Send certified.
P.S. I am not a lawyer.
What EVERYONE seems to be missing is that if the bank screwed up the paperwork, the previous owners rights should be AGAINST THE BANK NOT AGAINST THE PROPERTY. Individuals bought the property after it had been through the appropriate channels - they should not be at risk of having their home taken away from them based on a mistake that someone made in selling the property.