It is a possibility. They could have used these properties as leverage. Many investors do it, they buy an asset then use the value of that asset to borrow money to buy another asset or invest in a money making venture. They continue to use the money making asset to borrow more money to invest further. Some people accummulate dozens of properties by this method and owe a lot of money.
If the WTBTS had done this and used the borrowed money to invest in hedge funds they probably lost a lot of money during the financial crisis.
In Australia it is a pretty easy thing to do a title search on a property but it costs money. The title search shows if there is or was a mortgage attached at some time to a property, they are recorded on the title to establish third party interests. The WTBTS would probably be reluctant for this information to be made permanent and would be unlikely do this.