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NEW ORLEANS, Louisiana (CNN) -- New Orleans police lined up "like at a firing range" and fatally shot an unarmed man in the back as he fled from them in the days after Hurricane Katrina swept ashore, a witness to the shooting told CNN.

It marks the first time a witness has come forward publicly with information about the shooting of Ronald Madison, a 40-year-old mentally retarded man whose death has sparked a police investigation and a grand jury probe into what happened in and around the Danziger Bridge that day.

"He just fell like he was collapsing," Kasimir Gaston told CNN. "Like something just wiped him out." (Watch Gaston describe what he saw )

Gaston was one of many flood refugees living on the second floor of the Friendly Inn, a low-income motel on the city's east side. On Sunday, September 4 of last year, he says he woke up and stepped onto the balcony of the motel and saw a man running, hands outstretched and being fired upon.

Initial police accounts said that Madison reached for his waistband and turned on police, but Gaston said Madison did not appear to have a weapon and that he was running away from police "hands out, full speed" when he was shot.

Police declined CNN's request for an interview.

After the shooting last year, police said officers had responded to reported gunshots on the Danziger Bridge and that a running gunbattle ensued with six suspects.

One teenager was killed near the base of the bridge and three other people were wounded, according to police reports.

A police department press release from October 4, 2005, said Madison, described as an unidentified gunman, was "confronted by a New Orleans Police officer. The suspect reached into his waist and turned toward the officer who fired one shot, fatally wounding him."

When asked if Madison had a gun, Gaston said, "I didn't see any on him."

No gun was found on Madison's body.

An autopsy obtained previously by CNN and verified by the Orleans Parish Coroner said Madison suffered five gunshot wounds to his back and two in his shoulder. (Watch police describe a running 'gunbattle')

'What I saw with my own eyes'

Gaston said he came forward now because he is still troubled by what happened. He said he decided to break his silence after watching a "CNN Presents" documentary, "Shoot to Kill," about the days after Katrina hit the Gulf Coast on August 29, 2005.

CNN has visited the room where Gaston was staying. From that balcony, it is about 100 feet to where Madison was shot and killed.

The police department has said the shooting has been thoroughly investigated. But Gaston said no officer or detective approached him about what he saw. He was not asked for his name or phone number, Gaston said.

Gaston said his only contact with police on that day was when officers told him not to touch Madison's body, which was lying behind Gaston's truck, parked in the motel entranceway.

CNN has obtained a newspaper photo taken that Sunday morning that shows where the body fell. The back of a truck with a rusted trailer hitch and broken tail light can be seen in the photo. The photo appears to be Gaston's truck, which now sits in a parking lot in Dallas, Texas, where he now lives.

"They notified me that I had two bullet holes in the passenger side," he said. Two bullet marks can be seen at the right rear of the truck today.

Mary Howell, an attorney suing New Orleans police on behalf of the dead man's family, says there were several potential witnesses living at the Friendly Inn at the time of the shooting. She has accused police of violating procedures by failing to even write down their names.

Howell said Ronald Madison and his older brother, Lance Madison, were trying to avoid the shootout between police and others that day when they ran up the Danziger Bridge, toward the other side of the Industrial Canal.

Lance Madison has said a policeman pointed what looked like a rifle or shotgun at his brother and shot Ronald near the top of the bridge. Lance said he helped carry his wounded brother to the entrance of the motel and left him there while he ran for help. After being arrested, Lance was brought back to the motel where, he says, he first saw his brother was dead.

Howell said police, in the chaos after Hurricane Katrina, failed to properly investigate the fatal shooting and are now trying to put everything behind them.

"There's a lot of things that have been washed away with this hurricane," Howell said. "We are doing everything we can to make sure this is not one of them."

While the family of Ronald Madison presses on with its lawsuit, a grand jury in New Orleans is investigating the case. Gaston said he is willing to testify before the grand jury if it will help get to the truth.

"I'm not trying to say nobody did this and nobody did that," Gaston said. "I'm just saying what I saw and being truthful and honest about what I saw with my own eyes."