NEW ORLEANS  In its September newsletter, underneath a notice about using antibacterial wipes during flu season, the local Fraternal Order of Police reminded New Orleans officers they had a right to be represented by a lawyer when questioned by the F.B.I., whether as a witness or as a potential target.

The warning has become routine for the police over the last few months here.

Four years after the department was accused of acting lawlessly in suppressing violence in the wake of Hurricane Katrina, dozens of officers, some from an elite unit, have been interviewed by the F.B.I. or faced subpoenas to testify before federal grand juries. F.B.I. agents seized files from the department’s homicide division.

And on a recent Saturday, a major city bridge, where police officers killed two men in the floodwaters of 2005, was shut by federal investigators for several hours.

At least three federal investigations into the police are under way; two concern civilian deaths in the anarchic days after the hurricane. Their outcomes could answer some of the most disconcerting questions of the storm’s aftermath  in particular, whether these were singular tragedies in the extraordinary chaos of the time or whether they laid bare deep problems in the force.