WASHINGTON — A D.C. man was sentenced to 20 years behind bars for the brutal beating of his 81-year-old mother that led to her death in August.

On Friday, D.C. Superior Court Judge Zoe Bush sentenced 52-year-old Derek Cook to 20 years in prison for beating his mother, 81-year-old Ann Alfredia Cook, in their D.C. apartment on Aug. 12, 2017. The sentencing came as part of a plea agreement where Derek Cook pleaded guilty in September to a second-degree murder while armed charge.

Derek was having a conversation with his mother in the bedroom of their apartment on Aug. 12 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, according to evidence presented at the plea hearing.

Ann was unarmed in bed at the time of the beating.

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 had been in custody since the incident.

When Derek completes his prison term, he will be placed on five years of supervised release.