The police scandal that has roiled Chicago for months entered a new phase last week when a Cook County judge appointed a special prosecutor to investigate the cover-up that followed the police execution of a black teenager two years ago. The lawyers who petitioned the court for an outside prosecutor argued persuasively that the state’s attorney’s office, which would ordinarily handle such a case, had been effectively captured by the Police Department and was incapable of handling the matter objectively.

The special prosecutor, Patricia Brown Holmes, is a former judge and federal prosecutor, and clearly qualified for the post. But she will need steely perseverance to pry the truth from a Police Department that is well practiced in the art of concealing misconduct, brutality and even torture. This grim legacy dates back to the department’s infamous “midnight crew,” an outfit that battered and tortured black men from the 1970s into the 1990s.

This spring, a task force appointed by Mayor Rahm Emanuel found that the department was plagued by systemic racism and still operated with contempt for the lives of black citizens. The task force made many recommendations and proposed scrapping the current police oversight system in favor of a new investigative agency that would be under civilian control.

Mr. Emanuel created the task force shortly after the city was forced to release a police video of a white police officer, Jason Van Dyke, firing away at a black teenager named Laquan McDonald on a busy Chicago street. The city added to the African-American community’s already deep sense of grievance by withholding the video from the public for 13 months — while the police circulated a false account of the episode. Only after the court ordered the video released did the state’s attorney, Anita Alvarez, charge Officer Van Dyke with murder.