[Read: Chicago awaited the verdict with a “sense of anxiety.”]

For some residents, the trial became a proxy for years of anger over police mistreatment of black Chicagoans and over decades-old doubts about police accountability and transparency. They said they were relieved at the outcome and hopeful that it might force changes in policing and relations with city residents.

Image A family photo of Laquan McDonald. The teenager was shot 16 times in the fatal encounter. Credit... via Associated Press

Dashboard-camera video from a police car gave a clear view of the shooting, though the city for months resisted releasing the images and Chicagoans only saw it 13 months after it happened, on a judge’s orders. The fallout was significant: The police superintendent was fired, the local prosecutor lost her re-election bid, and Mayor Rahm Emanuel announced shortly before the trial began that he would not seek re-election next year.

“That video had a profound effect upon this city, not just on policing but on politics, and not just in black and brown neighborhoods — it rippled across every neighborhood,” said Lori Lightfoot, a former president of the Chicago Police Board, an oversight agency, who is now running for mayor. “People saw it and just said, ‘Dear God, what happened?’ and ‘What do we need to do so that that never happens again?’”

Police union leaders and supporters of Officer Van Dyke sharply criticized the outcome, and said it would have an instantly chilling effect on officers who were simply trying to do their jobs and stop crime. “This sham trial and shameful verdict is a message to every law enforcement officer in America that it’s not the perpetrator in front of you that you need to worry about, it’s the political operatives stabbing you in the back,” Chris Southwood, a state leader of the Illinois Fraternal Order of Police, said.

Chuck Wexler, executive director of the Police Executive Research Forum, suggested that the verdict could affect policing beyond Chicago, particularly when officers confront residents carrying knives and knifelike weapons. “Departments will be taking a second look at how they train officers to deal with individuals with edged weapons,” Mr. Wexler said.