Jason van Dyke fired 16 bullets into 17-year-old Laquan McDonald during the 2014 confrontation.
The video, some of the most graphic police footage to emerge in years, stoked outrage nationwide and put the nation's third-largest city at the center of the debate about police misconduct and use of force. Before he got there, McDonald had reportedly swung his knife at one witness, who had called 911.
At that point, Van Dyke shot McDonald and he fell to the ground.
Image copyright Getty Images Image caption Chicago police Officer Jason Van Dyke, left, lead away in handcuffs after his guilty verdict What happens now?
Three other officers await trial on charges of trying to cover up the killing and obscure the investigation. So, if Van Dyke gets the minimum for each count, he'd serve six years for all the battery convictions. The jury had eight men and four women, of those, seven were white, three Hispanic, one Asian-American and one African-American.
One juror noted as "the turning point" testimony that Van Dyke told his partner, "Oh my God, we're going to have to shoot that guy", before arriving at the scene. The judge told jurors the second-degree charge was also available, requiring them to find Van Dyke believed his life was in danger but that the belief was unreasonable.
"The video doesn't show my perspective", Van Dyke told prosecutors when asked about the discrepancy.
The city has been preparing or possible demonstrations in a high-profile case that already sparked protests.
The families of Van Dyke and McDonald were present in the fifth-floor courtroom.
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But, in a move not uncommon at IL murder trials, Judge Vincent Gaughan told jurors before they began deliberating that they could consider the charge of second-degree murder.
Van Dyke also was found guilty of all 16 counts of aggravated battery for shooting McDonald 16 times October 20, 2014.
First-degree required a finding that Van Dyke's use of deadly force wasn't justified that it was both unnecessary and unreasonable. Jurors found him not guilty of official misconduct.
Jurors said they spent much of their deliberations discussing whether to convict on first-degree or second-degree murder, not an acquittal.
"Someone needed to arrest Laquan McDonald, not stop him with a hail of gunfire", McMahon said in his closing argument on Thursday.
"I'm really happy", Curtis said, "I really felt like if they had returned a not-guilty verdict I would have burst into tears immediately".
Make no mistake about Cook County Judge Vincent Gaughan: He is in charge of Van Dyke's trial.
Schools and businesses braced for potential unrest, and people across the city paused in the middle of the day to listen for the jury's decision.
But Van Dyke's attorney, Dan Herbert, said the video, the centerpiece of the prosecutor's case, doesn't tell the whole story and is "essentially meaningless based on the testimony" jurors heard.
Anticipating violence in the wake of Friday's verdict, the Chicago Police Department has canceled overtime and placed an additional 4,000 officers on the streets.