WASHINGTON — A D.C. man pleaded guilty to charges that he beat his 81-year-old mother to death last month.

Derek Cook, 52, pleaded guilty Friday to a second-degree murder while armed charge after police say he brutally beat his mother, 81-year-old Ann Alfredia Cook, in their D.C. apartment on Aug. 12, 2017.

Police say Derek was having a conversation with his mother in the bedroom of their apartment in the 5000 block of Fourth Street NE when he began repeatedly hitting her with a closed fist. He also stuck her with a wooden stick and an iron on her face and her body.

Ann was unarmed in bed at the time of the beating, police said.

D.C. police arrived and found Ann severely injured in the apartment. She was taken to a hospital where she later died. Police arrested Derek, who has been in custody since the incident.

Cook’s guilty plea is contingent upon the court’s approval, and calls for an agreed-upon sentence of 20 years in prison. Cook’s sentencing was scheduled for Dec. 1, 2017.