The cover-up that began 13 months ago when a Chicago police officer executed 17-year-old Laquan McDonald on a busy street might well have included highly ranked officials who ordered subordinates to conceal information. But the conspiracy of concealment exposed last week when the city, under court order, finally released a video of the shooting could also be seen as a kind of autonomic response from a historically corrupt law enforcement agency that is well versed in the art of hiding misconduct, brutality — and even torture.

Mayor Rahm Emanuel demonstrated a willful ignorance when he talked about the murder charges against the police officer who shot Mr. McDonald, seeking to depict the cop as a rogue officer. He showed a complete lack of comprehension on Tuesday when he explained that he had decided to fire his increasingly unpopular police superintendent, Garry McCarthy, not because he failed in his leadership role, but because he had become “a distraction.”

Mr. Emanuel’s announcement that he had appointed a task force that will review the Police Department’s accountability procedures is too little, too late. The fact is, his administration, the Police Department and the prosecutor’s office have lost credibility on this case. Officials must have known what was on that video more than a year ago, and yet they saw no reason to seek a sweeping review of the police procedures until this week.

The Justice Department, which is already looking at the McDonald killing, needs to investigate every aspect of this case, determine how the cover-up happened and charge anyone found complicit. The investigation needs to begin with the Police Department’s news release of Oct. 21, 2014, which incorrectly states that Mr. McDonald was shot while approaching police officers with a knife. A dash cam video that was likely available within hours of the shooting on Oct. 20 shows Mr. McDonald veering away from the officer when he was shot 16 times, mainly while lying on the pavement. Why does the video completely contradict that press release?