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Mr. Letten is stepping down at a time when his office is ramping up an investigation of former New Orleans Mayor Ray Nagin’s administration. The probe already has resulted in guilty pleas by two businessmen who said they bribed an unidentified city official during Mr. Nagin’s tenure. It also has threatened to undermine other high-profile cases.

Last month, a federal judge ordered the Justice Department to conduct a new investigation of media leaks related to its probe of deadly police shootings on a New Orleans bridge after Hurricane Katrina. U.S. District Judge Kurt Engelhardt initially called for an investigation in June, but he said a report submitted by Ms. Mann is tainted and must be redone.

Judge Engelhardt presided over a trial in which five former officers were convicted of civil rights violations stemming from the Danziger Bridge shootings. Defense attorneys have asked Judge Engelhardt to order a new trial, arguing that media leaks deprived their clients of a fair trial.

Judge Engelhardt urged the Justice Department to appoint a court-approved independent counsel to investigate the online posts and media leaks. On Thursday, Mr.  Holder announced the appointment of John Horn, first assistant U.S. attorney for the Northern District of Georgia, to oversee the department’s new investigation of the matter.

Sen. Mary L. Landrieu, a Louisiana Democrat who has supported Mr. Letten even though he was initially appointed by a Republican president, said Mr. Letten’s record of rooting out public corruption should be commended despite the “troubling” allegations about his deputes. Mrs. Landrieu said she will forward a short list of candidates to replace Mr. Letten to Mr. Holder and the White House in the coming weeks.