CHICAGO (CBS) — A Cook County jury Tuesday found a South Side man guilty of murder for striking Fenger honors student Derrion Albert with a wooden board in a violently, frenzied melee that was captured on videotape.

The jury spent less than two hours before finding Eugene Riley, 20, guilty in the case.

“Each of those blows, each of those massive hits to Derrion Albert’s head, contributed to his death,” Asst. State’s Atty. James Papa said in his closing arguments earlier Tuesday.

Riley could have walked away when he saw Albert, 16, being repeatedly kicked in the head as he lay on the pavement near the Far South Side Roseland school on the afternoon of Sept. 24, 2009.

Instead, Riley further ignited the mob action by taking a large wooden board and slamming it down hard on Albert’s head, Papa said.

Riley, 20, testified Monday that he only hit Albert because he was trying to defend his younger brother, Vashion Bullock, who was knocked unconscious with a brick moments later.

“It was a reaction. I was scared. I didn’t know what was going on. I wasn’t thinking,” Riley said.

But Papa pointed out that Albert, bearing no weapons, had been knocked down and was being kicked and punched by others when Riley pounced on him.

“Eugene Riley’s involvement could have ended but it doesn’t,” Papa said. “… He finished him off.”

But Assistant Public Defender David McMahon said Riley, then 18, made a “mistake” and “never meant” for Albert to die.

McMahon also painted Albert as an instigator in the fight between Fenger students from Altgeld Gardens and an area known as “The Ville.”

Riley “was not part of that mob mentality,” McMahon said.

“The mob came to his brother. He came to his brother’s defense,” McMahon added. “He wasn’t going to leave his brother alone to that mob.”

McMahon also presented the videotape to the jury but pointed out that when played in slow motion, “it does not tell the whole story” and “distorts reality.”

But prosecutor Kathleen Bankhead said Riley, his brother and their friends “started the hurricane” when they stopped their car and got out to join the angry crowd on 111th Street.

It was Bullock, suspended for a fight at school earlier that afternoon, who proclaimed “it ain’t over” as he got ready to brawl, Bankhead said.

Bankhead said Riley could have taken his brother home. “He didn’t stop when he should have,” she said.

And with that Bankhead played the snippet of the video that shows Riley twice pelt Albert with the board one last time.

“That isn’t defending his brother. That wasn’t protecting himself. That’s first degree murder,” she said.

Riley’s friend, Silvonus Shannon, has already been sentenced to 32 years in prison for Albert’s murder.

In December, a juvenile boy was found delinquent, or guilty, in Albert’s murder, and in January, Eric Carson, 18, was sentenced to 26 years in prison after he pleaded guilty to murder.

A fifth man, Lapoleon Colbert, is awaiting trial.

(Source: Sun-Times Media Wire © Chicago Sun-Times 2010. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed.)