In the justice system, prosecutors have the power to decide what criminal charges to bring, and since 97 percent of cases are resolved without a trial, those decisions are almost always the most important factor in the outcome. That is why it is so important for prosecutors to play fair, not just to win. This obligation is embodied in the Supreme Court’s 1963 holding in Brady v. Maryland, which required prosecutors to provide the defense with any exculpatory evidence that could materially affect a verdict or sentence.

Yet far too often, state and federal prosecutors fail to fulfill that constitutional duty, and far too rarely do courts hold them accountable. Last month, Alex Kozinski, the chief judge of the United States Court of Appeals for the Ninth Circuit, issued the most stinging indictment of this systemic failure in recent memory. “There is an epidemic of Brady violations abroad in the land,” Judge Kozinski wrote in dissent from a ruling against a man who argued that prosecutors had withheld crucial evidence in his case. “Only judges can put a stop to it.”

The defendant, Kenneth Olsen, was convicted of producing ricin, a toxic poison, for use as a weapon. Federal prosecutors knew — but did not tell his lawyers or the court — that an investigation of the government’s forensic scientist, whose lab tests were critical to the case, had revealed multiple instances of sloppy work that had led to wrongful convictions in earlier cases. A state court found the scientist was “incompetent and committed gross misconduct.”

Yet the majority of the federal appeals court panel ruled that the overall evidence of Mr. Olsen’s guilt — including websites he visited and books he bought — was so overwhelming that the failure to disclose the scientist’s firing would not have changed the outcome.