Deputy Police Chief W.J. Riley said police shot at eight people, killing five or six.
Fourteen contractors were traveling across the Danziger Bridge under police escort when they came under fire, said John Hall, a spokesman for the Army Corps of Engineers.
They were on their way to launch barges into Lake Pontchartrain to help plug the breech in the 17th Street Canal, Hall said.
None of the contractors was killed, Hall said.
The bridge spans a canal connecting Lake Pontchartrain and the Mississippi River.
No other details, including whether any of the gunmen were killed, were immediately available.
The shootings come as the last weary refugees had been rescued from the Superdome and convention center and New Orleans turned its attention Sunday to gathering up and counting the dead across a ghastly landscape awash in thousands of corpses.
The bodies of those killed in Hurricane Katrina are everywhere: hidden in attics, floating in the ruined city, crumpled in wheelchairs, abandoned on highways.
"I think it's evident it's in the thousands," Health and Human Services Secretary Michael Leavitt said Sunday on CNN, echoing predictions by city and state officials last week about the death toll.
Craig Vanderwagen, rear admiral of the U.S. Public Health Service, said one morgue alone, at a St. Gabriel prison, expected 1,000 to 2,000 bodies.
New Orleans Mayor Ray Nagin said it was likely thousands were dead based on the city's population of 500,000, the percentage who left before the storm hit and the number evacuated from the shelters.
"So you probably have another 50-60,000 out there," Nagin said. "You do the math, man, what do you think? Five percent is unreasonable? Ten percent? Twenty percent? It's going to be a big number."
"We are still in the emergency. People must take seriously the fact that we have enormous ongoing challenges which we have to address right now or we're going to continue to have serious problems," Homeland Security Secretary Michael Chertoff Told CBS's Face The Nation.
Chertoddd warned the the scene was going to be as ugly as anyone can imagine.
He added rescuers going house to house have encountered a significant number of people who have said they do not want to evacuate, hindering relief efforts.
"We have to get them to shelter. We have to get them educated. We have to get them semi-permanent housing. We've got to dewater and clean up New Orleans. We've got an enormous challenge, and we've got one in Mississippi and in other parts of Louisiana and Alabama."
CBS News Correspondent Byron Pitts reports that at least 30 patients have died at the airport since Wednesday.
Sunday morning, a woman's body remained lying at the corner of Jackson Avenue and Magazine Street,, a business area in the lower Garden District with antique shops on the edge of blighted housing. The body had been there since at least Wednesday.
As days passed, people covered her with blankets or plastic.
By Sunday, a short wall of bricks had been built around her body, holding down a plastic tarpaulin. On it, someone had spray-painted a cross and the words, "Here lies Vera. God help us."
Leavitt said he had received a report of an outbreak of dysentery in Biloxi, Mississippi, with disease among the concerns of federal officials in the hurricane's aftermath. Dysentery is a painful intestinal disease that can cause dehydration and can sometimes be fatal.
The lack of clean drinking water in parts of the Gulf Coast region and standing flood waters with decomposing bodies and human waste in the streets of New Orleans could cause a rash of infectious diseases, including West Nile virus and the often fatal E. coli bacteria.
"All of the infectious diseases that occur when people are in large congregations of people can spread," Leavitt said.
Amid widespread criticism about a slow and ineffectual response to the crisis, the Bush administration dispatched several top officials to the region: Chertoff, Defense Secretary Donald H. Rumsfeld, Joint Chiefs of Staff Chairman Gen. Richard Myers and Secretary of State Condoleezza Rice.
President George W. Bush and first lady Laura Bush visited the Red Cross' disaster operation center, where they thanked employees. The president also announced that the White House would hold a blood drive on Friday.
Chertoff defended the job of Federal Emergency Management Agency Director Michael Brown and declined to get into a discussion about whether the government moved quickly and forcefully enough to deal with the catastrophe, saying there would be plenty of time for a review.
Chertoff told Face The Nation: "I think that the lesson of this hurricane, which we will clearly look at as we go over an after-action evaluation, is going to be very valuable in moving forward. I mean, this was an ultra-catastrophe, but we have to be prepared even for ultra-catastrophes, even things that happen once in a lifetime and once in a generation. So, yes, we will be studying that."