(CNN) -- Federal officials announced indictments Tuesday against four police officers and two supervisors in the investigation surrounding the post-Katrina deaths of civilians on New Orleans' Danziger Bridge.

At least three New Orleans police officers were in FBI custody Tuesday afternoon, an attorney for one of them confirmed. Kenneth Bowen, Anthony Villavaso and Robert Gisevius surrendered to authorities.

Announcement of the charges stemming from a federal civil rights investigation was made by Attorney General Eric Holder in New Orleans.

"Put simply, we will not tolerate wrongdoing by those who have sworn to protect the public," Holder told reporters.

Holder promised the Justice Department will help restore the troubled New Orleans police department.

"Today marks an important step forward in administering justice, in healing community wounds, in improving public safety and in restoring public trust in this city's police department," Holder said.

He was joined by the Justice Department's civil rights chief, Thomas E. Perez, and U.S. Attorney Jim Letten, the top federal prosecutor in New Orleans.

The shootings occurred at the bridge on September 4, 2005, six days after Katrina slammed into the Gulf Coast.

The indictment allegesthat two separate shootings at the bridge resulted in the deaths of a teenager and a 40-year-old disabled man.

The indictment contends four officers -- Bowen, Gisevius, Robert Faulcon and Villavaso -- opened fire, killing 19-year-old James Brissette. Minutes later, Faulcon allegedly shot and killed 40-year-old Ronald Madison. The indictment says the officers violated the victims' civil rights.

The indictment says police supervisors Arthur Kaufman and Gerard Dugue helped the other officers to obstruct justice during the subsequent investigations.

An earlier investigation launched by the local district attorney resulted in charges but no convictions. Federal prosecutors then moved in and launched a new investigation which led to the newly announced charges.

Letten said the federal case took so long because prosecutors waited for the local investigation to conclude, so as not to interfere or duplicate that probe.

Although the charges announced Tuesday carry a maximum sentence of the death penalty, Letten played down that likelihood, saying an extended process will eventually be followed and any final decision on seeking the death penalty would be made by Holder.

Attorney Frank DeSalvo, who represents Bowen, said of the indictment, "We expected it and are ready to deal with it."

The two shootings occurred on opposite sides of the bridge.

In the first shooting, on the east side of the bridge, one person -- later identified as Brissette -- was killed and four people were wounded, prosecutors have said. In the second shooting, on the bridge's west side, Madison, a severely disabled man, was killed. Madison's brother was arrested but later released without indictment, authorities said.

A witness to the shooting of Madison told CNN in 2006 that New Orleans police lined up "like at a firing range" and fatally shot the man in the back as he fled from them in the days after Hurricane Katrina swept ashore.

"He just fell like he was collapsing," Kasimir Gaston told CNN. "Like something just wiped him out."

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. He said on Sunday, September 4, 2005, 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, 40, 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 in response to Gaston's remarks.

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

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 report 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.

CNN's Terry Frieden contributed to this report.