In reality, Mr. Thompson’s case was one of many in which New Orleans prosecutors disregarded their constitutional duties. Louisiana courts have overturned at least 36 convictions as a result of these violations. And those are just the cases where wrongdoing was uncovered. Unfortunately, it’s all too easy for unscrupulous prosecutors to hide evidence that might hinder a conviction.

That’s what happened in the latest example of misconduct out of New Orleans, where prosecutors only last month disclosed a 19-year-old memo that undercuts their case against Robert Jones, who was sentenced to life without parole for a 1992 kidnapping, robbery and rape that he denies committing.

The case depended upon Mr. Jones’s connection to another man, Lester Jones, who was himself convicted of multiple crimes around the same time. Prosecutors at trial said the two men had been accomplices. But an internal memo written in 1996 makes it clear that prosecutors knew before trial that there was no evidence linking the men.

Despite their awareness of this key flaw, which the police had also alerted them to, the prosecutors did not reveal it to defense lawyers. Nor did the D.A.’s office turn over the memo when Robert Jones challenged his conviction a decade later, and Lester Jones reiterated in court that he did not know Robert Jones, and had told prosecutors this before trial.

The memo only came to light in late September, when a new prosecutor on the case found it and turned it over. By then, a state appeals court had already tossed out Robert Jones’s conviction because of other instances in which prosecutors withheld crucial evidence from the defense, including a statement from a witness describing someone who did not look like Mr. Jones.