AlliedInterstate Harassing debt collection calls for a debt I do not even owe. Warrenton Virginia

Several years ago, I had been using Washington Mutual as my bank. When I lost the job I had at the time, I closed my account, paying off all outstanding debts. Suddenly, in the last month or so, I've been receiving harassing phone calls and letters from a collection company called "Allied-Interstate" claiming I owe WAMU a huge debt, but not explaining where the debt came from, or how long it's been "in collection." I've gone back and checked, and double checked my records, and my credit report, and no such debt appears. I've contact Washington Mutual and they have no records of this debt either.