Chicago erupted three years ago when the city belatedly released a video showing that a white police officer had essentially executed a black teenager named Laquan McDonald and that the police and city officials lied about it for months. The public’s outrage drove the police superintendent and county prosecutor from their jobs. On Oct. 5, 12 jurors convicted the officer, Jason Van Dyke, of second-degree murder after less than eight hours of deliberation.

The candidates who are vying to succeed Mayor Rahm Emanuel — who announced in September, just before the trial began, that he is not seeking a third term in 2019 — need to know they could end up on the rocks if they ignore what the case teaches about police brutality and corruption. The verdict, the first time in nearly 50 years that a Chicago officer has been convicted of murder in an on-duty shooting, suggests that Cook County residents are no longer inclined to reflexively accept police lies about the use of deadly force.

The next mayor needs to end the city’s complicity with officers’ deceit and demand that police unions agree in contract negotiations to greater accountability and stricter oversight. Officials must commit to enforcing the elements of a consent decree with the state on police reforms whose final details are being hammered out in federal court.

The cover-up that began as McDonald lay bleeding on the pavement Oct. 20, 2014, is the subject of a second trial, set to begin next month. Three other officers are charged with lying about the shooting to justify Van Dyke’s actions, claiming McDonald had menaced officers with a knife.

Jurors in the Van Dyke trial could easily see from the video that McDonald, though armed with a knife, was veering away when Van Dyke shot him 16 times. Prosecutors also established that Van Dyke had essentially made up his mind to use deadly force even before he arrived at the scene. “We are going to have to shoot the guy,” he reportedly told his partner while en route.

Van Dyke’s testimony about that danger he faced held no sway. “We just didn’t buy it,” one juror said.

Top police and city officials may not have bought it, either, but they stood by the officers’ tales for months. Then, in April 2015, the city settled a case with McDonald’s family for $5 million, a move that prevented a trial at which the video could be shown. The city withheld the video for more than a year, releasing it in November 2015 only after a judge ordered it to do so.

The McDonald case — with its false statements, shoddy internal investigations and concealed evidence — is typical of how the department has historically operated. A 2017 Justice Department report, for example, found many cases in which the department had swallowed whole a police officer’s version of events that was later disproved by video evidence. The report also said that the city had failed to investigate a majority of the police misconduct cases it was required by law to examine, sometimes because of provisions in the union contract that reinforced a pervasive code of silence that has long existed among officers.

The McDonald case is not the only police corruption scandal in the public eye. The Cook County state’s attorney has vacated convictions in 42 cases — and is evaluating scores of others — because of illegal conduct by a corrupt former police sergeant who shook down drug dealers while running a protection racket in a South Side public housing project.

The pending consent decree, which the Trump administration opposes, would go a long way to changing how the department operates. It would require the city to create better supervision, new standards and record-keeping for the use of force, protection for whistleblowers and measures to prevent officer collusion during investigations.

But the decree, the result of a lawsuit brought against the city by Illinois Attorney General Lisa Madigan, also calls on the city to renegotiate provisions in the police union contract that discourage citizens who want to report misconduct while essentially encouraging officers to amend their initial statements — and even lie — during misconduct investigations. The city will obviously have to drag these concessions out of the politically powerful union.

The decades-old problems highlighted by the McDonald case will continue to undermine faith in law enforcement until the city commits itself to police reforms and makes sure that officers are subject to the rule of law.

FROM AN EDITORIAL IN THE NEW YORK TIMES