NEW ORLEANS — On Sept. 4, 2005, with floodwaters of Hurricane Katrina still standing in much of the city, Lt. Michael J. Lohman of the New Orleans Police Department arrived at the Danziger Bridge in eastern New Orleans. A group of police officers had rushed there just ahead of him in response to a radio call for assistance.

At the bridge, Lieutenant Lohman found that six civilians had been shot by police officers, two fatally. None of them had weapons.

Almost immediately, federal authorities said Wednesday in a blistering series of accusations, he and the other officers began to plot a cover-up, planting a gun near the site to make the shootings appear justified.

That action led to Lieutenant Lohman’s appearance in a federal courtroom on Wednesday afternoon, where he pleaded guilty to one count of conspiring to obstruct justice. It is the first charge in a wide-ranging inquiry into police misconduct that led to civilian deaths in the chaotic days after Hurricane Katrina, and it is unlikely to be the last.

“Know this,” Jim Letten, the United States attorney for the Eastern District of Louisiana, said to reporters after the hearing. “The investigation continues. It is ongoing.”

It is also not the only federal investigation into civilian deaths caused by the police force in the days after the hurricane. The fact that the accusations go beyond the shooting to a larger cover-up “is really indicative of a systemic integrity issue,” said Rafael C. Goyeneche III, a former New Orleans prosecutor who is the president of the Metropolitan Crime Commission, a watchdog group. “It’s going to rock the Police Department to the core.”

A spokeswoman for the Police Department declined to comment.

The bill of information, which contains charges but is not an indictment, and was unsealed Wednesday, is the clearest picture yet of the federal investigation into the Danziger Bridge shootings.

The documents filed by the authorities said that five of the civilians had been walking to get food and supplies, and that the other two were on their way to a family member’s dentistry office when they were fired upon by police officers. Four were seriously injured.

James Brissette, 19, and Ronald Madison, who was 40 and mentally disabled, were killed. Mr. Madison’s brother Lance, who was in the courtroom on Wednesday, was arrested and charged with eight counts of attempted murder in trumped-up charges related to the cover-up, but was later cleared.

Lieutenant Lohman, 42 and now retired, concluded shortly after arriving on the scene that the shooting was “legally unjustified,” federal authorities said. He encouraged the officers to “come up with a plausible story” that would allow him to conclude that the shooting was justified, the authorities said.

When another police investigator told Lieutenant Lohman that he was going to plant a gun under the bridge to bolster the story that the officers were being fired at, Lieutenant Lohman went along, and even asked if the gun was traceable, the authorities said.

At the encouragement of Lieutenant Lohman, the officers who were involved made up details, the authorities said, like a claim that one victim had reached for a “shiny object” in his waistband.

At one point, according to the documents, Mr. Lohman was frustrated that the cover-up story in the report, which was drafted by a police sergeant, “was not logical,” so he drew up one of his own, which broadly changed details to fit the false story. The sergeant later replaced that report with a shorter one that was changed to fit the audiotaped statements of the police who were involved.

That sergeant is unnamed by the authorities, but the police report is signed by Sgt. Arthur Kaufman, who was assigned to investigate the Danziger shooting. He prepared a supplemental report later with another police sergeant, which was widely criticized.

Sergeant Kaufman and at least one other officer, who was directly involved in the shooting, have received letters informing them that they are targets in the investigation, their lawyers acknowledged.

Lieutenant Lohman faces up to five years in prison, but Mr. Letten said the officer had been cooperating with the authorities, an indication that charges against others might be coming.

In 2006, the seven officers who were directly involved in the shooting were charged with murder and attempted murder, but the charges were dismissed in late 2008 by a judge who cited improprieties in the handling of the case. The United States Attorney’s Office, along with the F.B.I. and the civil rights division of the Justice Department, picked the case up soon afterward.

During the federal investigation throughout 2009, dozens of police officers testified before grand juries, federal agents seized files from the police homicide division, and the Danziger Bridge was shut down for hours as agents looked for evidence.

Several other cases are under investigation by the federal authorities, including the shooting death of 31-year-old Henry Glover, whose remains were eventually discovered in a burned car parked behind a police station in the Algiers section of New Orleans. That case was brought to the attention of the authorities by an article that appeared in The Nation magazine in December 2008 and on the Web site ProPublica.org.