JUAN SMITH WAS sentenced to a lifetime prison term based entirely on one man’s eyewitness account. On the stand, that witness, Larry Boatner, said that he was in a friend’s house in New Orleans on March 1, 1995, when Mr. Smith and several others burst in and demanded drugs and money from the occupants. Mr. Boatner testified that he had “no doubt” about his identification of Mr. Smith because he had been “face to face” with him at points throughout the crime. According to court documents, no other witnesses and no physical evidence implicated Mr. Smith, who was ultimately convicted on five counts of first-degree murder.

But statements previously made by Mr. Boatner to law enforcement officials undercut his confident testimony. On the night of the murders, notes taken by a detective show that Mr. Boatner “could not . . . supply a description of the perpetrators other [than] they were black males.” Five days later, Mr. Boatner again told the detective that he “could not ID anyone” because he could not see their faces and “would not know them [if he] saw them.”

The contradictory statements could have seriously, if not fatally, undermined the prosecution’s case, but jurors were never made aware of them. Law enforcement officials first failed to turn them over to Mr. Smith as the law requires, and then they only did so during Mr. Smith’s appeals.

Prosecutors’ failure to share exculpatory evidence — whether due to an oversight or cynical manipulation — is anathema to serving the cause of justice. Eight justices, in a Supreme Court decision written by Chief Justice John G. Roberts Jr., found the omission so troubling that they rightly erased Mr. Smith’s murder convictions.

The lone dissenter, Justice Clarence Thomas, essentially argued that the omissions should not disturb the convictions because there was no assurance that jurors would have reached a different conclusion. But the objective is not a guaranteed result but a fair process, and withholding exculpatory evidence grotesquely distorts that process.

This case once again sheds light on the troubling and long-running failures of the New Orleans district attorney’s office, which has been the subject of at least two recent Supreme Court cases because of its failure to turn over information favorable to a defendant. The Justice Department should bring its power to bear on the office to ensure it complies with the spirit and the letter of the law.