NEW ORLEANS — In Hurricane Katrina's chaotic aftermath, police shot six people – killing two – as they crossed a bridge in search of food. For years the case was a shocking symbol of the confusion and violence that swept through the flooded city. On Wednesday it became a mark of shame for the police department. As victims' relatives watched from the courtroom gallery, a retired lieutenant who supervised the department's probe of the shootings pleaded guilty to orchestrating a cover-up to conceal that police gunned down unarmed civilians. Michael Lohman, a 21-year veteran of the force, pleaded guilty to a charge of conspiracy to obstruct justice.