CHICAGO — Minutes after being convicted of murder, Chicago police officer Jason Van Dyke rose in the packed Cook County courtroom to be taken into custody. He had already said his goodbyes to his wife and family.

Looking fatigued but calm, the veteran patrol officer put his hands behind his back as his attorney, Daniel Herbert, clapped him on the back of his shoulder. As sheriff’s deputies escorted Van Dyke out of the courtroom, Herbert flashed him a thumbs-up.

With that, one of the most closely watched trials in Chicago’s history came to an abrupt conclusion.

A jury Friday convicted Van Dyke of second-degree murder and 16 counts of aggravated battery with a firearm in the death of 17-year-old Laquan McDonald, marking a stunning end to a racially tinged case that roiled the city when the now-infamous police dashboard camera video of the shooting was released three years ago by court order.

Van Dyke is the first Chicago police officer in half a century to be found guilty of murder for an on-duty shooting. He faces a minimum of six years in prison when he is sentenced by Judge Vincent Gaughan. The jury acquitted him of a single count of official misconduct.

Jurors told reporters most of them thought Van Dyke guilty when they entered deliberations but were initially split over whether to convict him on first- or second-degree murder. In the end, after deliberating for about 7½ hours, they settled on second-degree murder, finding that Van Dyke was frightened when he fired his gun but that his fear was unreasonable.

Around the city, a smattering of protesters gathered in public places to watch the verdict unfold. On the CTA Blue Line, riders packed cars heading from downtown while livestreaming the verdict on their cell phones for everyone to hear.

Outside the LaSalle Street entrance to City Hall, more than two dozen protesters formed a tight circle with their heads leaning in to listen to the decision.

Some covered their mouths and others braced anxiously with their hands resting on their head. A whisper rose among the hushed group, “I’m scared. I’m scared for my city, y’all.”

After several minutes, the crowd rejoiced in enthusiastic cheers after the first conviction was announced. Others like Keena Carson, an organizer and resident of Chicago’s Bronzeville neighborhood, broke into tears and doubled over with emotion.

“It just felt like all those years of work from the time the video came out was worth it,” Carson said. “Like all the organizing and being out in the streets, it was worth it. It was worth it — for once.

The slain teen’s sister, Tariana, wasn’t in the courtroom for the verdict but later posted on her Facebook page that the jury’s decision brought her “so many tears of joy.” Her next message was to her brother.

“I love you brother,” the 18-year-old Tariana wrote. “I miss you so much. I can sleep better because I know your killer isn’t walking the streets happy.”

Inside Gaughan’s fifth-floor courtroom, Van Dyke’s solemn expression did not break as the verdict was read. His wife, Tiffany, wept on word of his conviction. The slain teen’s great-uncle, the Rev. Marvin Hunter, seated on the other side of the courtroom, also cried.

Spectators remained silent, though some could be seen praying.

The judge had warned that any outbursts by those in attendance would result in their arrests. Deputies lined each corner of the packed courtroom.

Gaughan ordered Van Dyke’s bail revoked at the request of special prosecutor Joseph McMahon, who noted the officer faced the minimum of six years in prison.

“He now stands before this court a convicted felon,” said McMahon, the Kane County state’s attorney who was appointed to handle the prosecution because of Cook County prosecutors’ conflicts of interest.

Gaughan will likely set a sentencing date for Van Dyke when he returns to court on Oct. 30.

McMahon told reporters later that he regarded the second-degree murder conviction as appropriate justice for McDonald and his family. He said he had spoken to McDonald’s mother, Tina Hunter, after the verdict.

“This is a difficult day for Tina Hunter,” he said. “She has to continue to relive the worst moment of her life over and over. … I don’t think Tina will ever heal from this wound.”

Van Dyke’s lawyers vowed an appeal over the judge’s refusal to move the trial outside Cook County because of the extensive pretrial publicity.

“It really is a sad day for law enforcement,” Herbert said. “We’ve all heard about the Ferguson effect. I can only imagine if police officers think they can never fire against someone who is acting the way Laquan McDonald had when they’re 12 feet away from him, what are we going to have is police officers are going to become security guards. They are not going to want to go out and confront someone.”

The verdict comes after a landmark, three-week trial that flipped the script of most murder cases at the Leighton Criminal Court Building, with prosecutors questioning the credibility of police officers who typically serve as their most trusted witnesses.

Van Dyke himself broke from normal protocol for police officers facing charges of wrongdoing, opting to have a jury decide his fate instead of asking the judge to weigh the evidence in a bench trial. His decision to testify in his own defense also was a rare move, especially for one charged with murder.

The charges against Van Dyke centered on the dashcam video depicting the moments leading up to the shooting on Oct. 20, 2014 — footage that has been played around the world for nearly three years. The graphic images sparked protests and political upheaval and led to a sprawling federal civil rights probe into the systemic mistreatment of citizens by Chicago police, particularly in the city’s minority communities.

Throughout the trial, prosecutors highlighted how other officers involved in the incident operated with restraint, content to let McDonald walk away while they waited for backup cops with a Taser to arrive at the scene. One officer, in fact, trailed McDonald on foot for about half a mile over several blocks, never threatening to shoot. Van Dyke, however, opened fire just six seconds after stepping out of his squad car with his gun drawn. The car with the Taser arrived at the scene 20 seconds after he stopped shooting.

Five of Van Dyke’s fellow Chicago police officers testified for the prosecution, including Joseph Walsh, his partner that night who was granted immunity from prosecution while awaiting trial on separate criminal charges alleging he helped cover up details of the killing to make it appear justified.

The jury heard from dueling forensic pathologists who came to starkly different conclusions about McDonald’s death as well as two police use-of-force experts who disagreed on whether Van Dyke was justified in opening fire.

At times, witnesses called by one side wound up helping the other.

Walsh, for example, backed up Van Dyke’s version of events during his testimony as a witness for the prosecution, even stepping from the witness stand to demonstrate for the jury how he saw McDonald raise the knife to his shoulder in a threatening motion before his partner fired.

Perhaps more importantly, a psychologist paid by the defense testified that in the moments before Van Dyke had arrived on the scene, he told his partner that he might have to shoot the offender.

“Oh my God, we are going to have to shoot the guy,” Van Dyke recalled saying during an interview with psychologist Laurence Miller, according to Miller’s testimony.

Civilian witnesses also provided key testimony. A truck driver whose 911 call sparked the police response that night testified for the defense that McDonald tried to stab him when he caught him breaking into trucks. Two eyewitnesses — a father and son — testified for the prosecution that McDonald made no threatening movements toward the officer before he was shot.

The father, Jose Torres, the prosecution’s final witness in its case in chief, said all the shots fired at McDonald “upset me.”

“‘Why the f are they still shooting him when he was on the ground?’” he recalled himself asking out loud at the time.

But the case largely boiled down to the dashcam video that depicted the shooting as it unfolded as well as Van Dyke’s testimony in which he tried to explain his actions.

The video, played dozens of times for jurors over the three-week trial, showed Van Dyke and his partner pulling up to the scene as McDonald walked south in the middle of Pulaski Road, holding a 3-inch folding knife. Both jumped out with their guns drawn.

Six seconds after Van Dyke exited the car, he took a step toward McDonald — closing the distance to about 12 feet as the teen continued to walk at an angle away from him — and opened fire. McDonald spun and fell to the pavement.

Van Dyke continued firing for at least 12 seconds while McDonald lay prone in the street, emptying all 16 rounds into his body, prosecutors said.

Van Dyke told the jury that he was forced to make a split-second decision to shoot McDonald because the teen posed a threat and ignored commands to drop the knife.

But on cross-examination, he was unable to explain how he could have seen McDonald raise the knife moments before he opened fire when the video didn’t show it.

“What I know now and what I thought at the time are two different things,” Van Dyke testified.

Chicago Tribune’s Tony Briscoe contributed to this story.