NEW ORLEANS – The New Orleans Police Department, battered by criticism over its response to Hurricane Katrina and reeling from the resignation of its chief this week, announced Thursday that it had launched an investigation into officer misconduct in the chaotic days after the storm struck.

Police officers are suspected of standing by while looters emptied stores. Some are themselves suspected of looting. Witnesses saw police officers helped themselves to items from the shelves at a Wal-Mart in the lower Garden District.

It is the department’s second storm-related investigation. Police are also looking into officer desertion after the hurricane.

Acting Police Supt. Warren Riley said the misconduct investigation would be based largely on video evidence the department is collecting.

Video “does in fact show police officers with some items,” Riley said. “One video simply shows officers in a location where they were not in fact looting but weren’t stopping people who were looting.

“We have zero tolerance for misconduct from any member of this department,” he said. “Swift and decisive action will be taken.”

Riley said the department was functioning well and was able to handle the increased workload in the reawakening city. On Thursday, Mayor C. Ray Nagin opened the French Quarter, central business district and Uptown area to business owners. Today, residents of those areas will be allowed to return.

“The department is not dysfunctional,” Riley said. “We have 1,400-plus officers on the street. We are up and running.”

Riley replaced Supt. Eddie Compass, who stepped down Tuesday. At the time, a police spokesman said Compass was retiring to spend more time with his family. But Compass told his officers that the mayor forced him out, according to the New Orleans Times-Picayune newspaper.

The department was troubled long before Katrina. Federal officials recently completed an eight-year investigation into police abuse and civil rights violations. From 1994 to 1999, 200 police officers were dismissed or convicted of crimes, including two murders.

In the last month, the police have been criticized for doing too little to evacuate the city before the hurricane and to secure the city afterward. Anger at police grew as media began broadcasting scenes of the confusion and distress at the Superdome and convention center, along with rumors of violence that Compass helped spread in interviews.

The misconduct investigation currently focuses on 12 officers but will expand if necessary, Riley said. One officer has been reassigned, he said. He emphasized that police worked under highly stressful circumstances after Katrina, having lost homes and family members or being uncertain whether relatives had survived. He said both investigations might involve mitigating circumstances.

Video showed “that our officers did actually go in and get jeans, get food in some locations. We were without food for some days. Those things are acceptable; they’re acceptable to me,” Riley said. “If it’s something like jewelry or a television, that is not acceptable.”

Officers also commandeered Cadillacs from a car dealership. Officials said that was done because the storm destroyed more than 270 police cars, leaving officers unable to patrol.

“I can tell you there were some officers who did use Cadillacs,” Riley acknowledged.

Of the 249 officers being investigated for desertion, Riley said, “clearly not [everyone is] a deserter.” He said many officers were unable to contact the department after Katrina: Telephone and radio communications failed, and some officers were stranded on rooftops for four or five days.

“We had to rescue our police officers to get them back in,” Riley said.