Five New Orleans police officers have been convicted for their complicity in a series of murders and shootings of residents trying to escape the aftermath of Hurricane Katrina.

A jury found the five men guilty of covering up the killing of two people – including a mentally disabled man – and the wounding of four others in an elaborate plot that involved creating fake witnesses, falsifying police reports and statements, and planting a gun as evidence.

While the men escaped charges of murder, the jury in New Orleans also found Robert Faulcon, Robert Gisevius, Kenneth Bowen and Anthony Villavaso guilty of civil rights violations, while the fifth, Arthur Kaufman – a sergeant who investigated the shootings – was also guilty of taking part in the coverup.

"This was a critical verdict. I cannot overstate the importance of this verdict. The power, the message it sends to the community, the healing power it has," Jim Letten, the prosecuting US attorney, said after the verdict was announced.

The case is the most high profile of four major federal cases over the use of force by New Orleans police, and is likely to become a landmark civil rights decision.

The mother of 17-year-old James Brissette, who was killed in the city, welcomed the verdict after "a long, hard six years". But she questioned why the officers were not guilty of murder. "How are you able to empty a shotgun in the person and it's not murder?" Sherrel Johnson said to the Associated Press.

The shootings took place on the Danziger bridge over the city's industrial canal on 4 September 2005, as residents tried to find food and shelter in the chaos only five days after the hurricane had battered the city and destroyed its system of levees, flooding the inner-city streets.

The only one of the accused to testify, former police officer Faulcon, told the court he had been "paralysed with fear" when he shot and killed an unarmed 40-year-old mentally disabled man, Ronald Madison, in the back with a shotgun from his patrol car. But Faulcon claimed that he believed Madison was armed.

Prosecutors said Kaufman later produced a gun he owned and claimed it had been found at the scene, belonging to Madison's brother, and invented two nonexistent witnesses to the shootings. Sergeant Bowen was also charged with civil rights violations for kicking and beating Madison after he had already been wounded by Faulcon.

Bowen, Gisevius, Faulcon and Villavaso are all charged with civil rights violations for the shooting of Brissette, after the teenager was killed on the eastern side of the bridge, as well as for injuring members of another family.

Federal investigators reopened the investigation in 2008, which resulted in a series of guilty pleas by other New Orleans police officers involved in what was a systematic cover-up that began as soon as the shooting had stopped.