Chicago Mayor Rahm Emanuel announced the dismissal of Chicago police superintendent, Garry F. McCarthy, Dec. 1. Emanuel also said a police accountability task force is being established, following days of unrest over the alleged murder of a black teenager by a white policeman. (Reuters)

IT DIDN’T take long for Chicago Mayor Rahm Emanuel’s rickety one-bad-apple narrative to implode. As an explanation for a white police officer firing his gun 16 times into the body of black teenager Laquan McDonald in October of last year, it was a flimsy story from the outset given the systemic wrongdoing that allegedly followed the killing, and it quickly collapsed under the weight of popular outrage in the Windy City and across the country.

On Tuesday, Mr. Emanuel, facing irate questions from the public, the media and some of his political allies, beat a tactical retreat by firing Police Superintendent Garry F. McCarthy, naming a task force to address the police department’s obvious lack of accountability and acknowledging in a City Hall news conference what he called “the undeniable fact that the public trust in the leadership of the [police] department has been shaken and eroded.” Much the same is true about the mayor’s own leadership, as he also admitted.

A task force is well and good, but Mr. Emanuel, forced into a runoff and reelected in April, has more explaining to do.

The video, made public last week by order of a Cook County judge, shows Officer Jason Van Dyke opening fire even as the 17-year-old Mr. McDonald, who was carrying a small folding knife, veered away from police. The officer continued to shoot even after Mr. McDonald was on the ground, hitting him repeatedly in the back.

At Tuesday’s news conference, Mr. Emanuel defended the city’s efforts to keep the video secret as an effort not to taint the case — a case that showed no sign of life before the video was ordered released, at which point the Cook County prosecutor, Anita Alvarez, suddenly swung into action and, 13 months after the fact, charged Mr. Van Dyke with murder. (He has said he is innocent and will fight the charges.) In the face of the mayor’s dodging and weaving, Chicago reporters wanted to know, with good reason, whether he had fought the video’s release and delayed approval of a $5 million settlement with the McDonald family to protect his prospects in a tough reelection year.

A task force on police accountability cannot be the end of the McDonald case. At least seven other officers were standing there when Mr. Van Dyke opened fire; if they lied about it, they should be prosecuted too. Ms. Alvarez, the prosecutor, needs to explain her own inertia more convincingly than she has; if she cannot, she also should resign.

And what about the feds? FBI and Justice Department officials began investigating the McDonald killing just weeks after the fact. If Ms. Alvarez wouldn’t move to bring Mr. Van Dyke to justice, why couldn’t they? It’s not just that Chicago’s police force needs an overhaul (though it does); in the McDonald case itself, there must be accountability for the apparent lies, evasions and coverup that goes beyond the prosecution of one police officer.