CHICAGO (Reuters) - White Chicago police officer Jason Van Dyke was found guilty of second-degree murder on Friday for the 2014 shooting of black teenager Laquan McDonald, touching off celebratory street demonstrations in a case that had laid bare tensions between the black community and police.

Van Dyke, 40, was also convicted of 16 counts of aggravated battery, one count for each of the shots fired. McDonald, 17, was killed while armed with a knife as he walked down the center of a street in the third-largest U.S. city.

Jurors said they faulted Van Dyke for escalating the conflict when he could have waited for more police assistance, such as an officer with a non-lethal Taser weapon.

Van Dyke sat emotionless as the verdict was read. Cook County Circuit Court Judge Vincent Gaughan immediately revoked bail and Van Dyke was escorted out of the courtroom and into Cook County Jail.

He faces up to 20 years in prison for the second-degree murder conviction and up to 30 years for each of the 16 counts of aggravated battery.

His conviction seemed to quell any potential unrest of the kind that has occurred in other U.S. cities in recent years when white officers have been cleared of charges in the shooting deaths of black men.

“The end of this trial brings an opportunity for the community to come together,” special prosecutor Joseph McMahon told reporters.

Police killings of mostly unarmed black men elsewhere in the United States helped fuel the Black Lives Matter movement and became an issue in the 2016 U.S. presidential campaign.

Chicago had its own unrest when a police dashboard camera video was released more than a year after the shooting which occurred on the night of Oct. 20, 2014.

The video showed Van Dyke shooting McDonald as he walked down the middle of the street, veering slightly away from the officer. The aftermath led to the dismissal of the city’s police superintendent and calls for Chicago Mayor Rahm Emanuel to resign.

After Friday’s verdict, small, peaceful demonstrations assembled in central Chicago. About 100 people marched toward a gathering spot.

“Whose streets? Our streets!” some chanted. One protester bellowed into a megaphone: “This is what black power looks like.”

“We are in celebration mode but we also want to hold more people accountable,” said one of the protesters, Darius Parker, 28, who estimated the crowd would have been 10 times larger had Van Dyke been acquitted.

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The 12-person jury, which included one black person, began deliberations on Thursday afternoon.

After failing to reach a verdict on Thursday night, they began anew on Friday, finding enough mitigating circumstances to convict Van Dyke of second- rather than first-degree murder, jurors told reporters.

“In Van Dyke’s mind, he was doing the right thing. He was experiencing an extreme threat ... he needed to protect himself,” one man on the jury told reporters. The jurors who spoke to reporters were not identified by name.

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But jurors were swayed by the video, which prosecutors showed repeatedly during the three-week trial and which jurors reviewed during deliberations.

“We kept watching the video where he kept making the steps forward,” said another juror. “We watched that more than once. More than three times.”

Van Dyke testified in his own defense, saying he feared for his safety and fired because McDonald was advancing on him. Both the officer and his lawyers argued that the angle of the video did not reflect the incident from Van Dyke’s perspective.

But Van Dyke’s testimony seemed rehearsed and “we just didn’t buy it,” a third juror said.

Defense lawyer Daniel Herbert said he would appeal and push for a change of venue, which was denied to the defense in this trial.

“We knew coming into it with a Cook County venue in this case with the Cook County jury there was not a chance in the world we were going to come away with a complete not guilty,” Herbert told reporters.