PITTSBURGH (AP) – A man has been sentenced to 10 to 20 years in the stabbing death of his grandmother in western Pennsylvania four years ago.

Thirty-year-old Levi Staver was sentenced Monday in Allegheny County in the February 2013 death of 76-year-old Constance Johnston.

He pleaded guilty but mentally ill to third-degree murder in January, and prosecutors said his sentence will include significant psychiatric treatment.

Authorities said Johnston and her husband, a retired minister, were sitting at the kitchen table when Staver came upstairs from the basement and stabbed her in the back with a hunting knife. Police said Staver told them that an “archangel” told him to “kill the witch.”

Staver’s attorney sought a lesser term, but the judge expressed doubt that he would follow his treatment program if released from custody.

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