Have you ever wondered why it’s been so hard and slow to sort out the mortgage foreclosure mess in the U.S.?

Part of the reason can be summed up in two words: missing documents.

Two European banks have sued Bank of America over their mortgage investments, claiming they at least want back some of the mortgages that they allegedly invested in during the final years of the housing boom.

The probelm is no one can account for many of the mortgages in question. There’s alleged evidence some mortgages may have been sold twice. Some are now under the FDIC’s control due to bankruptcy filings by loan originators.

One mortgage portfolio — worth a half-billion dollars — reportedly has no paperwork at all.

The questions abound: Who did the borrowers pay each month? Who accepted the payments? Where did the money go? What happens if a borrower discovers his or her mortgage has no paperwork? Should they stop making payments and hope no one notices?

File under: Ghost mortgages