A white Chicago police officer was convicted of second-degree murder Friday in the 2014 shooting of a black teenager that was captured on shocking dashcam video that showed him crumpling to the ground in a hail of 16 bullets as he walked away from officers.

The video, some of the most graphic police footage to emerge in years, stoked outrage nationwide, and the high-stakes trial gripped the nation's third-largest city for nearly three years. The shooting also led to a federal government inquiry and calls to reform the Chicago Police Department.

Jason Van Dyke, 40, was the first Chicago officer to be charged with murder for an on-duty shooting in about 50 years. He was taken into custody moments after the verdict was read.

The second-degree verdict reflects the jury's finding that Van Dyke believed his life was in danger but that the belief was unreasonable. The jury also had the option of first degree-murder, a charge that required a finding that the shooting was unnecessary and unreasonable.

Second-degree murder usually carries a sentence of less than 20 years, especially for someone with no criminal history. Probation is also an option. Van Dyke was also convicted of 16 counts of aggravated battery — one for each bullet.

A man holds a sign with a photo of Laquan McDonald during protests in Chicago on Nov. 24, 2015. (Paul Beaty/Associated Press)

One legal expert predicted that Van Dyke will be sentenced to no more than six years total. But because he's an officer, it will be "hard time," possibly spent in isolation, said Steve Greenberg, who has defended clients at more than 100 murder trials.

The teen, Laquan McDonald, was carrying a knife when Van Dyke fired at him on a dimly lit street where he was surrounded by other officers.

One of Chicago's leading civil rights lawyers said the conviction sends a message to minority communities that the police reforms that began after the video became public were not just for show.

Andrew Stroth said an acquittal would have sent the opposite message, dashing hopes for change.

Demonstrators react to the verdict in front of George N. Leighton Criminal Courthouse in Chicago. (Nam Y. Huh/Associated Press)

Defence attorney Dan Herbert called Van Dyke "a sacrificial lamb" offered by political and community leaders "to save themselves." He said it was a "sad day for law enforcement" because the verdict tells officers they cannot do their jobs.

"Police officers are going to become security guards," he said.

A McDonald family spokesperson thanked prosecutors for pursuing a case that, he said, many black attorneys did not believe could be won.

"I can't rejoice because this man is going to jail," said McDonald's uncle, Rev. Marvin Hunter. "I saw his wife and father. His wife and daughter didn't pull the trigger. I could see the pain in these people. It bothered me that they couldn't see the pain in us."

Rev. Marvin Hunter, McDonald's uncle, speaks to the media at a church in Chicago after guilty verdicts were delivere on Friday. (Terrence Antonio James/Chicago Tribune via AP)

The verdict was the latest chapter in a story that has made headlines since a judge ordered the release of the video in November 2015. The case put the city at the centre of the national conversation about police misconduct and excessive force.

The 12-person jury included just one African-American member, although blacks make up one-third of Chicago's population. The jury also had seven whites, three Hispanics and one Asian-American.

Dispute over video

Officers were waiting for someone with a stun gun to use on the teenager when Van Dyke arrived, according to testimony and video. The video, played repeatedly at trial, showed Van Dyke shooting even after the 17-year-old was lying on the pavement.

Prosecutors and defence attorneys argued over what the video actually proved.

Prosecutor Jody Gleason noted during closing arguments that Van Dyke told detectives that McDonald raised the knife, that Van Dyke backpedaled and that McDonald tried to get up off the ground after being shot.

Watch: Fatal shooting of Laquan McDonald (edited version of graphic dashcam video)

Chicago police release a dashcam video that shows an officer shooting 17-year-old Laquan McDonald 16 times 1:49

"None of that happened," she said. "You've seen it on video. He made it up."

But Van Dyke and his attorneys maintained that the video didn't tell the whole story.

His attorneys portrayed Van Dyke as being "scared' by the young man who he knew had already punctured a tire of a squad car with the knife. Van Dyke testified that the teen was advancing on him and ignoring his shouted orders to drop the knife.

Van Dyke conceded that he stepped toward McDonald and not away, as Van Dyke had initially claimed. But the officer maintained the rest of his account, saying: "The video doesn't show my perspective."

'Blue wall of silence'

The officer had been on the force for 13 years when the shooting happened. According to a database that includes reports from 2002 to 2008 and 2011 until 2015, he was the subject of at least 20 citizen complaints — eight of which alleged excessive force. Though he was never disciplined, a jury did once award $350,000 US to a man who filed an excessive-force lawsuit against him. Van Dyke testified that McDonald was the first person he ever shot.

To boost their contention that McDonald was dangerous, defence attorneys built a case against the teenager, who had been a ward of the state for most of his life and wound up in juvenile detention after an arrest for marijuana possession in January 2014.

Among those testifying were several current or former employees at the Cook County Juvenile Temporary Detention Centre who said they had violent run-ins with McDonald. They also pointed to an autopsy that showed McDonald had the hallucinogenic drug PCP in his system.

Chicago police officers line up outside the courthouse on Friday. (Nam Y. Huh/Associated Press)

Prosecutors stressed that Van Dyke was the only officer to ever fire a shot at McDonald.

They called multiple officers who were there that night as they sought to chip away at the "blue wall of silence" long associated with the city's police force and other law enforcement agencies across the country. Three officers, including Van Dyke's partner that night, Joseph Walsh, have been charged with conspiring to cover up and lie about what happened to protect Van Dyke. They have all pleaded not guilty.

Even before the trial, the case affected law enforcement in Chicago. The city's police superintendent and the county's top prosecutor both lost their jobs — one fired by the mayor and the other ousted by voters. It also led to a Justice Department investigation that found a "pervasive cover-up culture" and prompted plans for far-reaching police reforms.

A week before jury selection, Mayor Rahm Emanuel announced he would not seek a third term, although his office insisted the case had nothing to do with his decision. He had faced criticism that he fought the release of the video until after his re-election in April 2015

Ahead of the verdict, the city prepared for the possibility of the kind of massive protests that followed the release of the video in November 2015, with an extra 4,000 officers being put on the streets.

The issue of race permeated the case, though it was rarely raised at trial. One of the only instances was during opening statements, when special prosecutor Joseph McMahon told the jurors that Van Dyke didn't know anything about McDonald's past when he encountered him that night.

"What we do know, what he [Van Dyke] did see, was a black boy walking down the street... having audacity to ignore the police," McMahon said.

Van Dyke's lead attorney Dan Herbert countered: "Race had absolutely nothing to do with this."

Watch: Race and Policing in Chicago (2016)