A District man pleaded guilty Friday to second-degree murder while armed in the Aug. 12 beating of his 81-year-old mother as she lay in her bed in the Northeast Washington apartment the two shared.

As part of the plea agreement with federal prosecutors, Derek Cook, 52, faces 20 years in prison when he is sentenced by D.C. Superior Court Judge Zoe Bush on Dec. 1.

According to charging documents, Cook fatally beat his mother, Ann Alfredia Cook, following an argument over the son smoking a cigarette in their apartment in the 5200 block of 4th Street NE. Cook admitted he beat his mother with an iron and a piece of a broken chair, the documents said.

The H. Carl Moultrie Courthouse is a courthouse of the Superior Court of the District of Columbia located at 500 Indiana Avenue NW, Washington, DC. (Keith Alexander/TWP) (Keith Alexander/TWP)

Cook also admitted to smoking crack cocaine twice earlier that day, according to the documents. He had his mother’s blood on his shirt when police arrived at the apartment to investigate the disturbance, the papers said.