A US judge has ordered a retrial in the case of five officers convicted of charges related to deadly shootings on a bridge after Hurricane Katrina.

The 2011 case was tainted by "grotesque prosecutorial misconduct" after three lawyers posted anonymous comments on a news website, ruled the judge.

The justice department said it was now considering its options.

A family member of one of the victims of the shootings said the decision had re-opened a "terrible wound".

Two unarmed men were killed by police on the Danziger Bridge in New Orleans, Louisiana, less than week after Hurricane Katrina struck in 2005.

'A bitter pill'

Five former city police officers co-operated with a justice department investigation and pleaded guilty to engaging in a cover-up to make the shootings appear justified.

The public must have absolute trust and confidence in this process Judge Kurt Engelhardt

Five other officers were tried on various charges of cover-up, firearms offences and civil rights violations.

Former US Attorney Jim Letten resigned in December 2012 after two top prosecutors in his regional office admitted posting anonymous comments about the case on nola.com, the website of the New Orleans Times-Picayune newspaper.

A wider investigation found a third lawyer, based in the justice department's civil rights division in Washington DC also posted anonymous comments on the website during the last week of the trial.

"NONE of these guys should had have [sic] ever been given a badge," one of the lawyers wrote under an assumed username.

"We should research how they got on the police department, who trained them, who supervised them and why were they ever been [sic] promoted."

Lawyers for the five police officers argued that the prosecutors' online comments and leaks to news organisations had been part of a "secret public relations campaign" that deprived them of a fair trial.

On Tuesday, Judge Kurt Engelhardt, who sentenced all five officers to terms ranging from six to 65 years, granted their request for a new trial, but called it a "bitter pill to swallow".

"The public must have absolute trust and confidence in this process," he wrote in his 129-page decision.

"Re-trying this case is a very small price to pay in order to protect the validity of the verdict in this case, the institutional integrity of this court, and the criminal justice system as a whole."