Looters, rogue cops, rifle-toting vigilante militias, and homes protected by jerry-rigged alarm systems made of empty strung-together beer cans.

New Orleans in the immediate wake of Katrina was a surreal, dismal, and sometimes deadly place.

Four years after the flood waters receded, it remains a city inundated by doubt about those days of chaos.

Did vigilante bands of roaming "people hunters" from a white neighborhood pass among the flood waters, shooting 11 black men, as one victim has alleged? A burnt car with a bullet-ridden body inside was found on the West Bank near the 4th District Police Station in the storm's aftermath. Police fired on civilians on the Danziger Bridge, thinking them looters, killing two.

What actually happened and who is culpable in these incidents is now the focus of probes by the US Attorney's Office and the FBI. Asked why investigations like these are just now being launched, New Orleans Mayor Ray Nagin recently explained, "We had a little event called Katrina."

Some New Orleanians believe the allegations point to "a broader pattern of violence ... that should reframe our understanding of the catastrophe," writes A.C. Thompson, a reporter with ProPublica, a nonprofit journalism venture.

In an article published in The Nation, a weekly news magazine, he claims that incidents of white-on-black violence "have gone unpunished."

But teasing the facts out of a flood of rumor and grains of truth will be daunting. It is a "morally treacherous" gambit to measure the actions of stressed people in the virtually lawless state of post-Katrina New Orleans by typical standards, says Peter Scharf, a criminologist at Tulane University. The outcome, he worries, could impact the willingness of first responders -- police, doctors and nurses -- to stay behind during a major natural emergency for fear of later repercussions.

The United States District Attorney and the FBI are "entering this incredibly gray, confusing period," says Mr. Scharf. "It's like investigating the Battle of the Bulge where everyone is lost in middle of the Ardennes Forest. There's ambiguity in the fog of war."

The New Orleans coroner's office has counted 23 dead with gunshot wounds to their heads, he says. What happened to these people is a mystery to authorities, he adds.

What happened next, Mr. Thompson alleges, amounted to mob justice. "[V]igilantes and residents -- citing the exact locations and types of weapons used -- detail a string of violent incidents in which at least eight other people were shot, bringing the total number of shooting victims to at least 11, some of whom may have died," he writes.

Several of the vigilantes have bragged of various shootings, but Loyola criminologist Dee Harper, for one, says "they don't come off as very credible -- something is missing there."

It's clear that footage from the time -- including a Danish documentary -- includes as much braggadocio as fact.

"I've heard that 120 people were executed and dumped in the river, but I've never seen proof that it actually happened," says Mr. Harper.

With the complete collapse of a functioning civil society after Katrina "rumors became urban legends," says former FBI agent Jim Bernazzani, who was on the scene.

One Algiers Point resident, Vinnie Pervel, said he does not appreciate being cast as "a thug."

He had been clubbed with a hammer by a looter the day after the storm, he told Thompson. Later, he came close to shooting somebody, but they ran off after being warned, he said. Mr. Pervel has been interviewed by the FBI.

"At the time, at the storm I thought, I guess, we could be considered like a neighborhood hero," he told CNN. "When the FBI contacted me, I felt like a vigilante, a thug."

Despite the difficulties involved with separating rumor from fact, the FBI has a responsibility to follow up credible leads, police sources say. FBI agents recently seized several New Orleans Police Department computers in a raid connected to an investigation of the Danziger Bridge incident.

A judge threw out indictments against seven police officers in the case.

"There's no presumption of guilt here," says Mr. Bernazzani, the former agent. "But the FBI will follow the facts. And if the facts warrant further scrutiny within the legal system, we have processes in place --because we are a nation of laws -- to achieve that end."