They were purchased by INVESTORS; ie Hedge Funds, Pensions, Investment Co's etc, they were never owned by WAMU.
WAMU likely retained residual interests in their MBS's but the vast majority were sold off.
What happened to those residual or any interests WAMU retained and what was their worth is still debatable.
What JPM bought essentially was the right to service those MBS's, they could not buy what didn't belong to WMB.