Danziger Bridge could have slipped back to being just a name on a map if the cover-up had succeeded. But as prosecutors accuse four New Orleans police officers of gunning down unarmed residents on its span, killing two, we know it will always stand for much more. Here the last bonds of trust between a city and its guardians died.

The alleged crimes occurred one week after Hurricane Katrina in 2005. The lasting shame may be that it has taken almost five years for federal charges to be filed. But their unveiling by the US Attorney General Eric Holder had the power and ominous shock of a thunderclap. The defendants, if found guilty, could spend the rest of their lives in prison – or they could face the death penalty.

Chaos came in several forms in the woebegone city in the days after the storm. Four-fifths was under water, hospitals lost power and public shelters were fouled by human waste. And there was a police department that became unhinged, nowhere more horrifically than on Danziger Bridge.

The indictments unsealed late on Tuesday allege what the families of the victims have asserted from the start. On 4 September 2005, the four officers fired on six civilians – going to shop for groceries and check on family property – without justification. Four were wounded – a woman had part of her arm blown off, her husband was shot in the head. Two were killed.

Even if the city has been reliving the tragedy for some time – state charges were filed against the defendants in 2006, but that case later fell apart – the details in the federal indictments still had the power to appal. They describe the shooting in the back of the mentally disabled man, Ronald Madison, as he tried to run away, and offer a new detail: after Madison fell, already mortally wounded, one of the officers stamped on his body.

Those facing possible death sentences are the police officers Kenneth Bowen, Robert Gisevius and Anthony Villavaso as well as former officer Robert Faulcon. They are charged not with murder but rather "deprivation of rights under colour of law". Along with two other officers, they are also charged with attempting to construct an elaborate cover-up that hinged on their assertions at the time that they had been fired upon on the bridge, which, it is alleged, they hadn't.

In court in New Orleans yesterday, three of the men – Mr Bowen, Mr Gisevius and Mr Villavaso, pleaded not guilty to all charges. Mr Faulcon was not in the courtroom, but is awaiting extradition from Texas to Louisiana to stand trial.

Eric Hessler, a lawyer representing Mr Gisevius, said that no consideration had been given to the chaos police were operating in during the first few days after Hurricane Katrina. "The federal government has clearly forgotten or chosen to ignore the circumstances police officers were working under," he told The New York Times.

According to the criminal indictments, the four officers arrived at the scene in a Budget rental van after receiving a radio message from a colleague nearby suggesting that officers may have come under fire from unidentified assailants. What they found when they got to the bridge was merely a straggle of pedestrians crossing from New Orleans to the other side.

Then, the complaint says, the firing began. James Brissette, 17, was with a friend's family heading eastwards in search of groceries. The group tried to hide behind a concrete divider on the bridge. The head wound and the partially lost arm were among the injuries they suffered. The bullets that hit James killed him.

Before he turned to flee, Ronald Madison was on his way with his brother, Lance, to check on the surgery of another brother, Rommel, a prominent New Orleans dentist. After the killing – and kicking – of Ronald, Lance was arrested by the officers. On him they hung their story of having come under fire. He was held for three weeks on charges of trying to kill a police officer before being released for lack of evidence, such as a gun.

That the four officers on the bridge could now be sent to Death Row, if found guilty, seemed yesterday to have left Rommel Madison – the brother of Ronald and Lance – in a quandary. "I don't want to see anyone executed, but I guess I have to keep in mind that they executed my brother," he said.

Tom Perez, the head of the US Justice Department's civil rights division, said that the indictments were "a reminder that the Constitution and the rule of law do not take a holiday – even after a hurricane".

But one New Orleans resident whose home virtually backs on to Danziger Bridge, gave voice to what many in the city still feel: they have a police force they cannot trust. "They put doubt in my mind about the police officers because, if they did that to those people, what they are going to do to us?" said Berthe Delonde.

The investigation into the Danziger shootings took two years and more indictments may still come. The breakthrough for prosecutors that makes the success of the federal case more likely came earlier this year when five former officers reached guilty pleas in connection with the alleged cover-up under which they undertook to offer testimony against the remaining defendants.

In the meantime, Mitch Landrieu barely had his feet under his desk this spring as the new Mayor of New Orleans when he announced that he had asked the federal government to investigate the city's police force "that has been described by many as one of the worst in the country". That review may result in a wholesale restructuring of the department.