CHICAGO (AP) — After deliberating for little more than half an hour, a jury convicted a 15-year-old boy of first-degree murder Wednesday in the 2009 beating of a Chicago high-school honor student that was captured on video and seen around the world.

The teen landed a single punch on the face of the other boy during a melee outside the school, but prosecutors argued that the punch “signed Derrion Albert’s death certificate.”

As the verdict was read, the teen’s aunt ran out of the courtroom yelling “Oh lord, Oh lord.”

The boy, who is not being identified because he is a juvenile, fell back in his chair and shook his head.

“I am pleased. Justice was served,” said Norman Golliday, Albert’s grandfather. “The facts were there from the start, they stared you right in the face. The jury saw that.”

But the convicted teen’s defense attorney said he would appeal. Lawyer Richard Kloak acknowledged that his client punched Albert as he staggered to his feet after being knocked on the head with a long board, but argued that the punch didn’t amount to the serious crime the teen was accused of.

“I wish the jury had been given a third option. They had to decide two choices, guilty or not guilty,” said defense attorney Richard Kloak. “I thought if I could give them a lesser crime that was proportionate to what he had done, it would be more palatable, but I didn’t get to do that.”

The teen is scheduled to be sentenced Jan. 18. He could be sent to prison until he is 21. But Kloak said that if he violates conditions of his sentence, he could receive 20 to 60 years in prison.

During the two-day trial, prosecutors contended that the then-14-year-old high school freshman was part of the mob that pummeled Albert with fists and feet, stomped on his head and struck him on the back of the head with a board, killing him.

The beating was captured on cell phone video and broadcast widely, providing the most vivid example of escalating violence that in a six-month period claimed the lives of more than 20 Chicago public school students. Albert’s death prompted President Barack Obama to send U.S. Attorney Eric Holder and Education Secretary Arne Duncan to the city to discuss ways to end the violence.

The court was crowded with friends of both Albert and the defendant. Albert’s mother wore a T-shirt with his picture. Some people left the court in tears Tuesday, the trial’s first day, as prosecutors showed the video, sometimes in slow motion.

The teen was among five people charged with murder in the case, but the other four were charged as adults. The first of them will go on trial in Cook County Criminal Court on Jan. 7, according to Kimberly Foxx, an assistant state’s attorney who supervises prosecutors in the case.

Prosecutors said the youth’s punch was thrown so hard that Albert wasn’t even able to put his hands up to break his fall, setting in motion Albert’s death.

“The defendant, with that punch, signed Derrion Albert’s death certificate,” said Assistant State’s Attorney Matthew Howroyd. The defendant “put Derrion in a position he could never recover from.”

Key testimony came Wednesday from a pathologist who conducted the autopsy, Dr. Hilary McElligot, who testified that the boy’s punch contributed to Albert’s death.

“Yes, I believe that punch was a contributing factor to his death,” McElligot said.

She added that it was impossible to tell which of the many blows killed him, but they all contributed to his death.

Kloak insisted that his client’s act ” did not cause the death.”

“He didn’t stomp anybody, hit anyone with a stick or hit anyone on the ground,” Kloak argued. “He may be guilty of something else (but) he is not guilty of first-degree murder.”

After Albert’s death, Mayor Richard Daley proposed initiatives including the deployment of more police officers to work in three-hour overtime shifts coinciding with school dismissal times, and beefed up the police presence at public transportation stops where students congregate.

Meanwhile, even as a couple of dozen students left Fenger after the Chicago Public Schools offered to find them another school, police launched a database that tracks daily incidents of violence near schools and even such mundane things as curfew violations.

CPS spokeswoman Monique Bond said that a number of programs are in place to help students get safely to and from public schools as well as such initiatives as conflict resolution programs for kids once they are inside the schools.

“I’m not aware of any incident of extreme violence, no shootings, since then (at Fenger),’ she said.

Copyright 2010 The Associated Press.