(Adds reaction from U.S. Attorney's Office)
By Kathy Finn
NEW ORLEANS, Sept 17 (Reuters) - A federal judge ordered a new trial on Tuesday for New Orleans police officers convicted of killing civilians in the chaotic aftermath of Hurricane Katrina, saying prosecutors had undermined the case by posting "vitriolic" comments about the police online.
Four policemen were sentenced last year to prison terms from 38 to 65 years for fatally shooting two unarmed people and wounding four others. A fifth officer was sentenced to six years for trying to cover up the crimes.
A senior U.S. Justice Department official last year called the case the most significant police misconduct prosecution since the black motorist Rodney King was beaten by Los Angeles police in 1991.
U.S. District Judge Kurt Engelhardt, in ordering the new trial, said that before and during the trial, prosecutors had used assumed names to post online messages criticizing the New Orleans police and their defense team. The messages appeared in a forum on www.nola.com, a website affiliated with the New Orleans Times-Picayune newspaper.
The culprits were revealed last year after people involved in an unrelated case hired private investigators, who traced the online comments to the prosecutors in the U.S. attorney's office, resulting in two resignations.
In a 129-page ruling, Engelhardt blasted the prosecutors and quoted from messages posted by Sal Perricone, one of those who resigned.
"Perricone, under his several monikers, habitually posted comments portraying the NOPD, its superintendent Warren Riley, and its officers and personnel in the most negative and vitriolic way," Engelhardt said.
The judge quoted from messages that called then-police Superintendent Riley - who testified in his officers' defense - "inept" and "delusional."
"The publication by DOJ (Department of Justice) employees of inflammatory invectives, accusatory screeds, and vitriolic condemnations ... should confound and alarm any reasonable observer of the criminal justice process," Engelhardt wrote.
Though the prosecutors who resigned were not directly involved in the case, the judge wrote that "the use of the media, in ways that might very well prejudice defendants, and create an overriding tenor of guilt in the community long before trial, must be avoided."
The prosecutors' behavior placed the court in an "unacceptably awkward position," Englehardt said.
The U.S. Justice Department issued a brief statement following the ruling: "We are disappointed with the court's ruling. We are reviewing the decision and considering our option," a spokesperson said.
(Reporting By Kathy Finn; Editing by Greg McCune and Gunna Dickson)