Last month, ProPublica, The New Orleans Times-Picayune and PBS "Frontline" joined forces to examine a string of violent encounters between police and civilians that occurred in the days after Hurricane Katrina made landfall. Officers shot at least ten citizens, killing a minimum of four. Our series focused on several questionable shootings, and found the department initially made only cursory efforts to determine whether officers acted properly.

Now our partners at The Times-Picayune are reporting that New Orleans Police Department Superintendent Warren Riley and former Superintendent Eddie Compass testified Friday before a federal grand jury. For more on what this means, check the story here.

While it's unclear what police officials discussed while in court -- grand jury proceedings are secret -- in recent months federal prosecutors have focused on two key cases involving the NOPD: the Danziger Bridge incident, during which police shot six people, and the death of Henry Glover, who apparently died while in police custody.