CHICAGO (CBS) — A 15-year-old boy convicted in the beating death of Fenger High School student Derrion Albert was sentenced to a juvenile detention center until he turns 21, the maximum sentence for a juvenile.

If the unidentified teenager breaks any rules while in juvenile detention, he must serve 30 years in prison as an adult, Cook County Juvenile Court Judge Colleen Sheehan ruled Tuesday.

Sheehan acknowledged that the 15-year-old hadn’t meant to seriously harm honor student Derrion Albert in the Sept. 24, 2009, attack that was caught on a cellphone video that was widely seen on the Internet.

But, “Even though you didn’t intend to do that, you are responsible for his death,” Sheehan said

The teenager, 14 at the time of the attack outside Fenger on the Far South Side, could be released from juvenile detention before age 21 but must serve at least five years behind bars under terms of his sentence.

He can be seen on the video punching Albert in the face, sending him crashing to the pavement.

Albert died after being punched, kicked and struck by wooden boards in broad daylight.

The 15-year-old was found “delinquent” — guilty in juvenile court terminology — of murder last month.

Last week, a Cook County jury found another suspect in Albert’s death, Silvonus Shannon, guilty of murder. Three others are awaiting trial.

On Tuesday, Albert’s mother, Anjanette Albert, told the teen that he “stole” her gift when he killed her son. “You helped destroy a family.”

The boy apologized to the family.

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

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