A District man was sentenced Friday to 15 years in prison for beating to death the 20-month-old son of his then-girlfriend, who had left the child with him while she was at work.

D.C. Superior Court Judge William M. Jackson sentenced Steven Cephas, 22, in the Nov. 8, 2010, death of the toddler, Dominic Kingsbury Jr.In September, Cephas pleaded guilty to voluntary manslaughter.

According to prosecutors, Cephas was playing video games at an apartment in the 5000 block of Jay Street NE, and he became angry when Dominic awakened. Cephas severely beat the boy, hit him in the head and threw him against a wall, authorities said.

About 12:30 a.m. Cephas called his girlfriend, telling her that Dominic had stopped breathing. The boy’s grandmother, Denise George, rushed to the apartment and found Dominic lying face up with bruises and marks on his face and stomach. After being taken to the hospital, Dominic was pronounced dead. An autopsy found that the boy had suffered blunt force injuries, hemorrhages, lacerations of the intestine and liver, and spinal cord contusions.

At a highly emotional hearing Friday, Dominic’s family, including his mother, father, aunts and uncles, sat in four rows on one side of the courtroom. Cephas’s family sat in two rows on the other side. Several of Dominic’s relatives rushed out of the courtroom sobbing as Assistant U.S. Attorney Cynthia Wright described the killing.

“This child was punched repeatedly,” Wright said. “There was blood on the walls. This innocent child was pummeled.”

George, Dominic’s grandmother, stood a few feet from Cephas, who was shackled and standing with his attorneys from the District’s Public Defender Service.

“My grandson was my heart. He was my first grandson. My grandson was my everything,” George said through tears. “I would like to ask why. Why did you brutally murder my grandson? Why did you murder my grandson?”

Cephas did not respond.

At the time of Dominic’s death, Cephas was facing prison time for violating the terms of probation from a case in which he was convicted of assault with a deadly weapon and carrying a pistol without a license. On July 29, 2010, another Superior Court judge, Herbert B. Dixon Jr., ordered Cephas to serve seven years in prison after the probation violation.

Dixon told Cephas to surrender to marshals, but Cephas fled the courthouse.

At the hearing Friday, Jackson ordered Cephas serve the 15-year manslaughter sentence after the seven years he was already serving.