...as an FYI for investors, banks that packaged and sponsored (sold) mortgage backed securities (bought by investor trusts by payment to the sponsor selling bank), don't get paid for them a second time (i.e., not coming back).
...the banks, WMB and subs, do have retained interests in such if the mortgages in the series and tranches perform better than prospectus; and such is recorded and fair valued on the balance sheet and income statement monthly.
...as such, they "are on the books."
...they are not "off balance."
...and such assets of WMB were seized as per the above.