The officer testified that he feared for his life as he approached McDonald, who was walking erratically down a street on the city’s Southwest Side with a small knife in October 2014. An autopsy found that the teenager had PCP in his system at the time.

Van Dyke’s lawyers said the teenager would be alive if he had dropped the weapon as police instructed. Prosecutors argued that McDonald’s death was not justified and accused Van Dyke of “exaggerating the threat”. Jurors found Van Dyke not guilty of official misconduct.

Aislinn Pulley, a founder of Black Lives Matter in Chicago, called the verdict historic because of the rarity of a police officer being “held accountable for murdering a black person in this city”.

“The beginning of a change is possible,” she said.

After the verdict on Friday, demonstrators took to the streets to celebrate the outcome. Hundreds of marchers shut down North Michigan Avenue from Millennium Park to the Chicago River.

Van Dyke faced a first-degree murder charge, which carries a maximum sentence of life in prison. But William Holloway, the activist whose lawsuit forced the courts to release dashboard-camera video of the shooting, said he was not upset by the jury’s decision to convict Van Dyke on the lesser second-degree murder charge.

“Murder suffices,” he said. “Anything less than murder … would not be justice.”

Jurors who spoke to reporters after the verdict said an acquittal was never on the table. One said he believed that Van Dyke, an officer on the force for nearly 20 years, should have been aware there were nonfatal tactics available to subdue McDonald.

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“Instead of escalating the situation, he should have de-escalated,” the juror said. Jurors names were not released.

Jeff Neslund, the lead lawyer for McDonald’s family, said the family was “relieved” by the verdict. He said the teen’s mother, Tina Hunter, planned to speak at the sentencing hearing. She has largely stayed out of the spotlight, declining requests for interviews.

“This will be a time for the mother to express her outrage,” Neslund said. “She’ll be able to pour her heart out to the judge.”

The verdict is the latest fallout from McDonald’s death, a case that dominated the second term of Chicago Mayor Rahm Emanuel. After the footage of the incident was released in November 2015, Emanuel ousted his police superintendent, Garry McCarthy, who later said he was a fall guy and is now running for mayor. Voters then dismissed the prosecutor in the case, who waited a year to charge Van Dyke. The Justice Department launched an investigation, concluding in a scathing report last year that the department uses unreasonable, excessive force and violates residents’ constitutional rights.

A week before Van Dyke’s trial began, Emanuel announced he would not run for a third term.

Van Dyke’s defence lawyer, Dan Herbert, said Friday’s verdict marked “a sad day for law enforcement”, suggesting police officers would now hesitate to fire their weapons in potentially dangerous situations.

Herbert said he felt the evidence did not support the jury’s determination, but added that he was relieved Van Dyke was not convicted of first-degree murder.

“He’s standing,” he said of Van Dyke. “He’s a tough man. He knew the stakes. He knew the climate. We knew coming into it, with a Cook County venue in this case [and] with a Cook County jury, there was not a chance.”

Special Prosecutor Joseph McMahon rejected Herbert’s suggestion that the verdict would cause police officers to not take action when threatened because they feared potential litigation.

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“That’s not reflective at all of the hundreds of law enforcement officers I know,” McMahon said. “I don’t think the verdict will interfere with that obligation one bit.”

Instead, he hopes it will create “a new chapter in the relationship between the law enforcement and the community”.

Outside the courthouse on Friday, Fraternal Order of Police President Kevin Graham blamed the city for making Van Dyke a scapegoat. He suggested that if there were more officers on the street and equipment like tasers in every vehicle, Van Dyke would not have had to use his gun.

“We believe he acted as a police officer and did the best he could that night,” Graham said. “It’s a shame the equipment and manpower was not there when he needed it.”

McDonald’s death – which came just weeks after black teenager Michael Brown was killed by a white police officer in Ferguson, Missouri – did not draw nationwide attention until 13 months later, when the court ordered the release of the police dash-cam video.

Authorities had initially said McDonald lunged at police officers, but the footage showed McDonald walking down the middle of Pulaski Road before hitting the ground when he was struck by Van Dyke’s bullets. The police department has recommended firing officers for lying about McDonald’s death, and three current or former officers were indicted last year on charges of conspiring to cover up what happened.

Van Dyke remains an inactive officer on unpaid leave, the department said Friday, pending the Chicago Police Board’s review of the recommendation from Eddie Johnson, the police superintendent, that he be fired.

Tensions surrounding the Van Dyke trial were high. Protesters gathered outside the courthouse every day of the three-week trial. Some high-rise owners in downtown Chicago warned residents of potential violence following the verdict.

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The trial has drawn particular scrutiny in part because of how rarely on-duty officers are charged for fatal shootings. Convictions are even less likely to follow, as officers have wide latitude under the law to use deadly force. In recent years, fatal shootings of civilians by police officers in Cincinnati, Milwaukee, North Charleston, South Carolina, and the Minneapolis area have spurred intense protests, followed by criminal charges. Most of those cases ended with acquittals or deadlocked juries.

The verdict in Chicago is among the exceptions – the second time in recent months that a jury has convicted a police officer in a controversial shooting. In August, a Texas jury found former Balch Springs officer Roy Oliver guilty of murder for the April 2017 shooting of Jordan Edwards, a 15-year-old black boy who was sitting in the passenger seat of a car that was driving away from a house party. Oliver was sentenced to 15 years in prison.

An ongoing Washington Post database found that on-duty police officers fatally shoot just under 1,000 people each year. A Post analysis of on-duty shootings between 2005 and 2015 found 54 police officers who had been charged in connection with such a shooting. Of those officers, 16 were convicted.

In a statement after the verdict, Johnson and Emanuel called for police, officials and residents alike to “continue to hear each other and partner with each other”.

“While the jury has heard the case and reached their conclusion, our collective work is not done,” they said. “The effort to drive lasting reform and build bonds of trust between residents and police must carry on with vigour.”

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After the video’s release in November 2015 – the same day Van Dyke was charged with first-degree murder – protesters marched on downtown Chicago streets throughout Emanuel’s term. They gathered in front of the mayor’s home, on Lake Shore Drive and even on two lanes of the Dan Ryan motorway to call attention to police reform and what they charged was a cover-up by city hall.

City officials had fought releasing the tape in court; it was made public only after Emanuel won re-election to a second term and after Chicago’s City Council approved a US$5 million settlement with the McDonald family.

Last year, the city borrowed US$225 million for police-related settlements and judgments, bringing the total to US$709 million between 2010 and 2017, according to a report by the Action Centre on Race and the Economy. The organisation estimates that the borrowing will eventually cost Chicago taxpayers more than US$1 billion in interest for the life of the bonds.