In principle there are a few possibilities - foreigners, pension funds, banks, insurances or other corporations, endowment funds, publicly owned,... although most of these would seem more likely in a place like the US.
It's also conceivable that nobody really knows, that there is no common land registry that really matches the reality 'on the ground', ie the people considering themselves owners and receiving rents. In such a situation it would be very hard to make transactions, hence why it might appear impossible to buy real estate in some areas.
