AFTER Hurricane Katrina, the flooded city of New Orleans sank into chaos. Wild rumours spread, of civil strife and bloodshed. The mayor, disgracefully, fanned them. And the police, scared and ill-disciplined, committed terrible acts.

The worst took place on the Danziger Bridge, where officers responding to a report of a fellow cop under fire shot six unarmed civilians, killing two of them. Rather than owning up, the officers covered up, framing an innocent man, planting a gun and inventing witnesses. In 2011, six years after the storm, a federal court found five officers guilty of civil-rights crimes or orchestrating a cover-up; four received multi-decade sentences. (Five others had already taken plea deals.)

But on September 17th Kurt Engelhardt, a federal judge, declared a mistrial, citing “grotesque” misconduct by prosecutors, some of whom had posted pseudonymous comments about the trial on a news website, thus prejudicing the case.

The scandal broke in March 2012, when a wealthy landfill owner who was the target of a federal corruption probe claimed that a prosecutor was commenting online under the handle “Henry L. Mencken1951.” The prosecutor quickly resigned, but that was not the end of the story. Months later, the same landfill owner charged that another prosecutor in the office was also an online bloviator. Soon, that prosecutor was also gone, as was her boss, Jim Letten, at the time the longest-tenured of America’s 93 US attorneys.

A special prosecutor, John Horn of Atlanta, was appointed to dig deeper. Some of the fruits of his inquiry surfaced this week. Perhaps the most startling: a prosecutor in the Justice Department’s civil-rights division, which oversaw the Danziger case, had also been a regular online commenter on the case.

Mr Engelhardt, who had earlier expressed discomfort with the government’s antics but was reluctant to order a new trial, said that that news may have been the “proverbial straw that broke the camel’s back.” The judge was peeved that Mr Horn had initially withheld the prosecutor’s identity from him, which Mr Engelhardt saw as part of a larger attempt at a whitewash by the Justice Department.

Was the prosecutors’ misconduct serious enough to require a new trial? Mr Engelhardt cited questionnaires asking jurors their opinion of the New Orleans Police Department. Those who regularly read NOLA.com, the website where prosecutors left their barbed commentary, had a slightly dimmer view of the police than those who didn’t, he noted. He added: “The time may soon come when, some day, some court may overlook, minimise, accept or deem such prosecutorial misconduct harmless ‘fun’. Today is not that day.”

The decision left relatives of the Danziger victims stunned. “What’s going to happen to the crimes they committed?” Sherrel Johnson, whose 17-year-old son was killed on the bridge, asked the New Orleans Advocate. “Are they just going to sweep that under the carpet and forget it?”

That is unlikely. But the prosecutors’ postings have jeopardised a big civil-rights prosecution and torpedoed the FBI’s pending corruption probe involving the landfill owner. Other cases, pending and settled, could be vulnerable to similar challenges, as prosecutors’ online snark was not confined to the Danziger case. A new local US attorney, Kenneth Polite, was confirmed by the Senate on the same day as the Danziger verdicts were thrown out. He has a tough job ahead of him.