This is what Chicago is up against. This is why trust in the police is low.

This is why the only right call for Mayor Rahm Emanuel is to accept federal court oversight of a historic effort to reform the Chicago Police Department.

We want to believe justice is blind in Chicago, but every so often a challenge to that comforting notion explodes to the surface, and it happened again on Thursday. In a secret FBI report finally unsealed in federal court, a former Cook County prosecutor describes how Chicago police detectives and assistant state’s attorneys unethically colluded in 1994 to charge four young African-American men with rape and murder.

EDITORIAL

The four men each spent 15 years in prison before DNA tests in 2011 linked the crime to an entirely different suspect, Johnny “Maniac” Douglas, a murderer and sex offender shot to death in 2008. A judge threw out the convictions against the “Englewood Four” and later granted them certificates of innocence.

In the five-year-old report released Thursday, former Assistant State’s Attorney Terence Johnson, under questioning by the FBI, describes how county prosecutors buckled under pressure from the police to press charges against the Englewood Four, failing to carry out their own properly skeptical assessment of the evidence.

One supervisor, Johnson said, pushed him to “move the case along” and “always wanted to give the detectives whatever they wanted.” The supervisor, he said, accused any prosecutor who balked of not “being a team player” and assigned him or her to some second-tier task as a punishment.

Detectives “coached and fed” witnesses and pressured the defendants with false assurances, Johnson said, and the collusion continued even after charges were filed. Before a court hearing, he said, a detective passed around a cheat sheet specifying how each cop and prosecutor should testify so that their stories lined up about how confessions in the case were secured.

Johnson said the detectives complained to his boss when he insisted they get more evidence, and he said he never really bought the cops’ argument that the woman, 30-year-old Nina Glover, was killed because she owed the defendants money. But when Johnson complained to a colleague about the cheat sheet, he said, he was warned to forget about it if he cared about his career.

Whenever cops and prosecutors behave so wrongly in Chicago, defenders of the status quo dismiss it as a problem of “a few bad apples” and insist it was a one-off. But this is not true. It is systemic and continual, and that is why broad reform of the police department — including in its dealings with the state’s attorney’s office — is so overdue.

It was not a one-off when police detectives and the state’s attorney’s office bent over backward in 2004 — and again in 2011 — not to file criminal charges against a nephew of then-Mayor Richard M. Daley in connection with the death of 21-year-old David Koschman. The nephew, R.J. Vanecko, had punched Koschman after a night of drinking.

It was not a one-off on Oct. 20, 2014, when as many as eight police officers witnessed a fellow officer, Jason Van Dyke, shoot 16 bullets into a teenager, Laquan McDonald, but claimed to have seen or heard no evil. In their witness reports, they described a version of reality — Van Dyke, in legitimate fear for his life, fending off a knife-wielding attacker — that a video showed to be nonsense.

Terence Johnson’s description of the collusion between detectives and prosecutors has a strong air of truth about it, though Johnson has credibility issues of his own. In 2000, he was convicted of felony sexual abuse of a minor. The fact remains that he describes what old hands at the Leighton Criminal Court Building say is common practice.

Mayor Emanuel is facing pressure from all quarters to accept, in the form of a consent decree, federal court oversight over a plan to reform the police department. Illinois Attorney General Lisa Madigan and former U.S. Attorney Zach Fardon recently joined the chorus. Doing so would remove any hint that local politics is tainting this essential work.

It is not enough to reform the Chicago Police Department. People must believe the reform is real.

Send letters to: letters@suntimes.com.