FIVE DAYS after Hurricane Katrina leveled the Gulf Coast in 2005,New Orleans police officers responded to what turned out to be false reports that a fellow officer had been shot. Arriving at the Danziger Bridge on the city’s east side, they jumped from their vehicles and opened fire on two groups of unarmed civilians. When the officers’ barrage was finished, two men were dead and four were badly wounded.

The 2011 conviction of five of the officers was a victory for federal prosecutors and, more important , for justice in a city where it often has been elusive and subverted. That’s why a federal judge’s decision last week to vacate those convictions is a dereliction of justice far greater than the prosecutorial abuses he cited in his order.

In throwing out the officers’ convictions , U.S. District Court Judge Kurt D. Engelhardt said he did not take such an action lightly. He paid lip service to the sanctity of the jury’s 2011 verdict, the considerable cost of the original trial and a new one (if it becomes necessary ) and the immense emotional toll already sustained by the victims, the defendants and their families.

The judge argued that the case had been perverted and the outcome poisoned by what he called “grotesque prosecutorial misconduct” on the part of two federal lawyers in New Orleans and one in Washington. Their transgressions involved posting anonymous online comments before and during the trial (in many cases attacking the police department) on, the Web site of the New Orleans Times-Picayune.

Judge Engelhardt is right that the online comments were egregious, unjustifiable, unprofessional abuses of authority on the part of the lawyers. The two prosecutors in New Orleans left their jobs in disgrace. The Justice Department lawyer, whose role was revealed in the judge’s decision, may face disciplinary action. Conceivably, they could be disbarred.

However, his conclusion that the online postings created a “prejudicial, poisonous atmosphere” that justified throwing out the convictions is a huge stretch. By that logic, overturning the convictions might also be justified by the TV show “Treme,” which began airing on HBO 14 months before the officers’ trial and depicts the New Orleans police as corrupt, brutal and violent. It’s a safe bet that more New Orleanians have seen “Treme” than the prosecutors’ online postings.

As the judge himself acknowledged, there is no evidence that members of the jury saw the online postings in question or any online postings about the case. (Once chosen as jurors for the trial, they were all under the judge’s orders not to read anything about the case.) In the howling wind of pre-trial publicity about the case — in print, online, on television and in social media — it is far-fetched to believe that the online rants of the prosecutors, while blatantly improper, were anything more than a speck of dust.

Judge Engelhardt’s emotional 129-page ruling is unconvincing in the extreme. It gives the impression that he is so exasperated and infuriated with prosecutors, for a host of reasons not confined to the online postings that he has thrown out the officers’ convictions in a fit of pique. The result is to restore what many people in New Orleans surely wished had receded along with Katrina’s floodwaters: an abiding sense that justice has not been done.