A cell phone video of the beating was played in court.
The video shows a group of teenagers attacking Albert. The prosecution says the teenager on trial threw one of the first punches.
Albert's family was in court as the trial got under way.
The teen is not being identified because he is a minor. His attorney says that although this client punched Albert, he did not inflict any harm that caused Albert's death.
The final moments of Albert's life were captured on that notoriously violent video which was played in court over and over again.
The teenager charged with beating Albert to death sat quietly and watched the cell phone video that captured the September 24, 2009, melee. It was played in court four times -- three times in slow motion.
Each time, muffled sobs could be heard from Albert's mother and relatives, as they watched the violent images of the 16-year-old getting attacked.
Jurors were also visibly shaken.
The first viewing was during the testimony of T'awannda Piper, a youth worker at a Far South Side community center. Piper said how she witnessed the fight outside the doors of the center and saw the teen on trial, who was 14 at the time, punch Albert in the face, knocking him down.
Albert had been trying to get up after being hit on the head with a wooden plank.
Piper broke down while explaining how she ran outside and helped bring Albert into the center.
During Jamal Harding's testimony, the 19-year-old admitted to throwing punches in the mob and identified each of the four other defendants charged in Albert's death who were on the videotape and are now awaiting trial. They are Silvonus Shannon, Eugene Riley, Eric Carson and Lapoleon Colbert.
The video that circulated the web and captured the nation's attention shows a fight among at least 50 teenagers stemming from a long-standing rivalry between two groups of students at Fenger High School.
Albert was an honors student at Fenger.
If convicted, the teen could be sentenced to prison until he's 21. If he violates conditions of his sentence, he could receive 20-to-60 years in prison.
The trial could wrap up Wednesday.
