Updated: 12/02/10 1:16 p.m.

JOLIET, Ill. (CBS) – A southwest suburban Mokena teen pleaded guilty to first-degree murder Thursday in the killing of his 4-year-old half sister.

Keith Randulich, 19, of the 19400 block of York Drive, was arrested May 22, 2009, in the stabbing death of Sabrina Clement.

Randulich called 911 about 8 p.m. that day. When police arrived, they found the girl’s body in the basement.

The girl suffered what was described in court today by Assistant State’s Attorney Mike Fitzgerald as “numerous and massive stab wounds to her neck”, according to a release from the Will County State’s Attorney’s office.

The teen allegedly told police at the time that he had killed the girl because someone was abusing her. Because he couldn’t kill the abuser, he killed his half sister instead.

He took the girl to the basement of his home and stabbed her to death with a steak knife while she was lying on the floor. His parents were not home at the time.

The Will County State’s Attorney’s Office will argue that the crime was brutal and heinous when prosecutors ask the judge to hand down a life sentence.

“This was a crime of unspeakable cruelty and horror,” said State’s Attorney James Glasgow. “One cannot begin to fathom what would prompt a young man to commit such an atrocity against his little sister.”

Randulich will be sentenced Jan. 25. He could face a sentence of 20 to 100 years in prison, or life in prison without parole.

The Sun-Times Media Wire contributed to this report.

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