A COURT DECISION may be legally correct but ultimately fall far short of justice. That is the case with the decision rendered last week by the Supreme Court in Connick v. Thompson .

Prosecutors are required to hand over evidence to the defense that may prove exculpatory, an obligation first articulated in the 1963 case of Brady v. Maryland . But prosecutors in the Orleans Parish (La.) District Attorney’s Office inexcusably withheld blood evidence that could have proved — and ultimately did prove — that John Thompson was not the culprit in an armed robbery. Mr. Thompson was later convicted of murder in a trial in which he declined to testify on his own behalf for fear that the robbery conviction would be used to impeach him.

Mr. Thompson spent 18 years behind bars — 14 on death row — and came within weeks of being executed. His fate might have been tragically different but for the diligence of his lawyers and a private investigator who discovered the withheld evidence. Louisiana judges threw out his convictions; when New Orleans District Attorney Harry Connick Sr. retried Mr. Thompson on the murder charge, a jury returned a verdict of not guilty after just 35 minutes. Mr. Thompson was paid the maximum $150,000 — a relative pittance — from a state compensation fund, according to his lawyer.

Those who are wrongly incarcerated often sue individual prosecutors for egregious violations that led to the injustice, but Mr. Thompson could not, because the prosecutor in his case — he confessed to a friend after learning that he was terminally ill — was dead by the time of the exonerations. Mr. Thompson instead sued the district attorney’s office for failing to properly train attorneys.

Mr. Thompson and his lawyers contended that the district attorney’s office should have known that without robust training, its prosecutors were bound to botch their obligations to share exculpatory evidence. A five-justice majority of the high court, in an opinion written by Justice Clarence Thomas, concluded that Mr. Thompson had not met his very high burden of proof. In the process, the court overturned a $14 million judgment in Mr. Thompson’s favor.

Law enforcement officials and offices should not live in fear of being on the hook for millions of dollars for good-faith errors; such a burden could chill their ability to make tough but legitimate calls. But individual prosecutors who break the rules should be disbarred and held personally liable in the most egregious cases when they have violated clearly established constitutional rights. States should also consider adopting “open file discovery” rules that enable defense counsel to view virtually all of the evidence in the prosecutor’s possession — leaving it up to defense counsel to determine which evidence favors their client.

Mr. Thompson, who has secured funding for a foundation to help other wrongly imprisoned individuals, acknowledges what seems apparent in this case — that the gross injustice he suffered was most probably caused by a prosecutor who knew the law and deliberately flouted it. “I don’t think training would have had anything to do with nothing really, to be honest with you,” he said, according to the New Orleans Times-Picayune, “because you could have trained them and they would still do it.”