A veteran New Orleans police officer pleaded guilty yesterday to orchestrating an elaborate cover-up of a shooting in the days after Katrina in which police gunned down six unarmed city residents, killing two and seriously wounding four.

The development — which the Times-Picayune calls a “potentially devastating blow” to other officers linked to the case — is the first plea in a wide-ranging federal probe of several post-Katrina police shootings. The Feds are reportedly looking at possible crimes in both the shootings themselves as well as the subsequent investigations.

The Danziger Bridge shootings occurred on Sept. 4, 2005, just six days after Katrina hit. The victims were reportedly stranded on a part of Chef Menteur highway that was surrounded by flooding; the police involved were working out of a temporary station at a reception hall nearby.

In federal court Wednesday, former Lt. Michael Lohman plead guilty to one count of conspiracy to obstruct justice in the incident. He arrived on the bridge after a group of officers had opened fire at two groups of unarmed citizens, leaving Ronald Madison, a mentally disabled 40-year old man, dead from six gunshot wounds, and James Brissette, 19, dead from seven gunshot wounds.

In charge of supervising the police investigation, Lohman drafted a bogus 17-page report on the incident, and helped other officers prepare false reports that would agree on a single narrative. When another investigator planted a handgun at the scene, Lohman questioned him to make sure it was “clean.”

Lohman admitted “he knew that the civilians on the bridge had not actually possessed guns, and he knew that the investigator had also falsified statements by the civilians,” according to the government’s press release.

The seven police involved in the shooting itself claimed that they had come under fire after responding to a call that reported two officers had been shot in the area of the bridge, according to an account of the incident from ProPublica. The outlet has been investigating the incident along with the New Orleans Times-Picayune and Frontline.

After the police shooting, officers arrested Lance Madison — brother of one of the slain men — and booked him on attempted murder charges, alleging he had shot at officers. A grand jury later rejected those charges, according to the Times-Picayune, which has a useful timeline of the case here.

What actually happened, according to the Feds, is this: at least seven officers arrived at the bridge in a Budget rental truck. For unclear reasons, the police opened fire when they encountered on the east side of the bridge a group of six people heading to a supermarket to get supplies. That’s when Brissette was killed; Susan Bartholomew lost part of an arm, and her husband Leonard was shot in the head.

Then the officers traveled to the west side of the bridge and encountered the Madison brothers, who were headed to visit a family member’s dentistry office. Ronald Madison, who lived with his mother and, according to his brother, had the mental capacity of a young child, was shot five times in the back, an autopsy showed.

The police involved in the incident were out of uniform and heavily armed, according to the Times-Picayune. They later claimed that one of the victims had reached for a “shiny object,” according to the New York Times.

The seven officers involved in the bridge shooting were indicted on murder charges in December 2006, but the judge threw out the case in August 2008, citing a prosecutor’s violation of grand jury secrecy. The next month, U.S. Attorney Jim Letten launched a probe of the case.

It’s not clear whether the officers involved in the shooting itself could still face federal charges.

ProPublica has a lot more here on other questionable police shootings in the wake of Katrina.