JOLIET, Ill. (CBS) — A teenager from Mokena, who admitted to cutting the throat of his 4-year-old half sister, was sentenced to 40 years in prison on Thursday.

Keith Randulich, 19, pleaded guilty last December to the murder of Sabrina Clement in 2009.

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He was arrested May 22, 2009, after he called 911 from his home in the 19400 block of York Drive. When police arrived, they found the girls’ body in the basement.

The girl suffered what was described in court as “numerous and massive stab wounds to her neck,” according to Will County prosecutors.

Randulich, who turns 20 on Saturday, told police he slit her throat to protect her from another abusive relative, but the Will County Coroner said there was no evidence of molestation on her body.

Before killing the girl, Randulich said he wanted to buy a gun to kill the “bigger and stronger” abuser, but his mother refused to purchase a firearm for him.

So Randulich waited for his parents to leave the home, grabbed a steak knife and lured the girl into his basement bedroom. After he killed her, he ran up the stairs and called 911 to report the crime.

“I gave her a kiss and said, ‘I am sorry, I had to do it’,” he told police.

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

(The Sun-Times Media Wire contributed to this report.)

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