Chicago: A white Chicago policeman who shot a black teenager to death was charged with murder on Tuesday in a prosecution hastened in hopes of averting renewed racial turmoil over the use of lethal police force that has shaken the United States for more than a year.

A highly anticipated video of the 2014 shooting, taken from a camera mounted on the dashboard of a police squad car, was released under orders from a judge hours after the officer in question, Jason Van Dyke, made his first appearance in court.

Amid intense interest from the media and public, the police department website where the footage was posted became overwhelmed and initially crashed.

The city braced for demonstrations but so far only a group of about 100 people were protesting about a mile south of Chicago’s business district.

The video clip showed 17-year-old Laquan McDonald, who authorities said was carrying a knife and had the hallucinogenic drug PCP in his system, as he was gunned down in the middle of a street on 20 October, 2014, as walked away from officers who confronted him.

From the dashboard camera, two police officers are seen jumping out of their car and drawing their weapons while advancing toward McDonald. Within seconds he is struck by bullets and crumples to the ground, his body jerking as he is hit by additional rounds of gunfire.

Prosecutors said he was shot 16 times by Van Dyke, who emptied his gun and was preparing to reload his weapon. Van Dyke has said through his lawyer and the police union that the shooting was justified because he felt threatened by McDonald.

‘Clearly, this officer went overboard and he abused his authority, and I don’t think use of force was necessary,’ top Cook County prosecutor Anita Alvarez said at a news conference after Van Dyke’s initial hearing.

Chicago Mayor Rahm Emanuel appealed for calm as the city prepared for possible protests.

‘It is fine to be passionate but it essential to remain peaceful,’ Emanuel told a news conference to announce the release of the video. Emanuel was flanked by a dozen community leaders.

At the same news conference, Police Superintendent Garry McCarthy said police would ‘facilitate’ protests but would not tolerate criminal behaviour.

Van Dyke was denied bail at a hearing in Chicago’s main criminal courthouse hours after prosecutor Alvarez announced charges of first-degree murder. If convicted, Van Dyke could face 20 years to life in prison.

At the brief court hearing, Cook County Circuit Court Associate Judge Donald Panarese scheduled a second hearing for Monday where he said he wanted to view the video in court and reconsider bail for Van Dyke based on its content. Prosecutor Bill Delaney told the judge that witnesses and the video concur that McDonald was not moving toward Van Dyke.

Misconduct

Van Dyke has had 20 misconduct complaints made against him during the past 4-1/2 years, none of which led to any discipline from the Chicago Police Department, according to research by Craig Futterman, a University of Chicago law professor and expert on police accountability issues.

‘The Chicago Police Department refuses to look at potential patterns of misconduct complaints when investigating police misconduct,’ Futterman said. ‘If the department did look at these patterns when investigating police abuse, there is a great chance right now that 17-year-old boy would still be alive.’

He believes Van Dyke is the first Chicago police officer to be criminally charged for an on-duty shooting.

Alvarez also said prosecutors moved up the timing of the charges ahead of the release of the video.

‘With release of this video it’s really important for public safety that the citizens of Chicago know that this officer is being held responsible for his actions,’ she said.

Last week, a court ordered the release of the video. The police union objects to its release.

McDonald’s death came at a time of intense national debate over police use of deadly force, especially against minorities. A number of U.S. cities have seen protests over police violence in the past 18 months, some of them fuelled by video of the deaths.

The uproar was a factor in the rise of the Black Lives Matter civil rights movement and has become an issue in the 2016 U.S. presidential election campaign.

Van Dyke’s lawyer Daniel Herbert said his client would prevail in court.

‘This is a case that can’t be tried in the streets, it can’t be tried in the media, and it can’t be tried on Facebook,’ Herbert said.

Family calls for calm

McDonald’s family called for calm, as did city authorities and black community leaders.

‘No one understands the anger more than us, but if you choose to speak out, we urge you to be peaceful. Don’t resort to violence in Laquan’s name. Let his legacy be better than that,’ McDonald’s family said in a statement through their lawyer.

In Baltimore and Ferguson, Missouri, family appeals for peace were not always heeded.

Black community leaders in Chicago said they feared violent protests in reaction to the video. Politicians and church leaders in the Austin neighbourhood urged potential demonstrators to protest peacefully.

‘We feel your pain, but we challenge you to turn your pain into power. We know protests are coming, please allow them to be peaceable,’ the Rev. Ira Acree said at a news conference.

Police shootings are frequent in Chicago, the third-largest city in the United States with 2.7 million people, roughly one-third white, one-third black and one-third Hispanic.

From 2008-2014 there were an average of 17 fatal shootings by police each year, according to data from the Independent Police Review Authority, which investigates police misconduct.

Almost all shootings, fatal and non-fatal, are found to be justified.