ST. LOUIS (AP) — A Missouri man already serving 20 years in state prison for killing his wife has pleaded guilty to a federal charge expected to add more years to his time behind bars.

Clay Waller pleaded guilty Thursday to interstate domestic violence. He pleaded guilty in 2013 to second-degree murder in a deal with prosecutors that allowed him to avoid the death penalty.

He admitted killing Jacque Sue Waller in 2011 and burying her body on Devil's Island on the Illinois side of the Mississippi River near Cape Girardeau, Missouri. He and his wife — the parents of triplets — were divorcing at the time.

The U.S. attorney's office in St. Louis says the federal plea agreement calls for a 35-year sentence if accepted by a judge on Jan. 16. The federal and state sentences would run concurrently.