One case is that of a former Algiers resident, Roland J. Bourgeois Jr., who is white and was accused of being part of one of the vigilante groups. He was recently indicted by the federal government on civil rights charges in the shooting of three black men who were trying to leave the city. According to the indictment, Mr. Bourgeois, who now lives in Mississippi, warned one neighbor that “anything coming up this street darker than a brown paper bag is getting shot.”

The highest-profile case involving the police is the Danziger Bridge shooting in eastern New Orleans, where six days after Katrina, a group of police officers wielding assault rifles and automatic weapons fired on a group of unarmed civilians, wounding a family of four and killing two, including a teenager and a mentally disabled man. The man, Ronald Madison, 40, was shot in the back with a shotgun and then stomped and kicked as he lay dying, according to court papers.

Mayor Mitch Landrieu in May invited the Justice Department to conduct a full review of the city’s Police Department. The Justice Department has also begun several civil and criminal investigations into post-Katrina violence involving the police and civilians.

Thomas Perez, an assistant attorney general, said the federal government was investigating eight criminal cases involving accusations of police misconduct. Many people in the city — including activists, victims and witnesses — had long contended that racial violence was being ignored by local law enforcement.

“We were dismissed as kooks for the last four years,” said Jacques Morial, a co-director of the Louisiana Justice Institute, a nonprofit advocacy organization, and the son of New Orleans’ first black mayor. “I think what we are seeing now recalibrates the reality of Katrina, and I think it vindicates lots of folks.”

The city’s police superintendent, Ronal Serpas, who took over the department in May, said he was troubled by what has come to light since the storm.

“We have to confront this and look at it head on,” Mr. Serpas said. “There have been far too many examples of men who have worn this badge and admitted in court to behavior that is an absolute insult to this city and to the men and women of this department who wear this badge with dignity and pride.”