Federal prosecutors in Minneapolis this week questioned the U.S. attorney over the lack of any civil rights investigations into two fatal shootings by immigration agents, and warned that more people could resign in protest if things don't change, multiple sources told CBS News. The meeting comes as the FBI is asking agents who have experience handling assault on federal officer cases to volunteer to head to Minneapolis, several different sources said. Since then, another six assistant U.S. attorneys have also tendered their resignations, sources say. Justice Department officials are now bracing for the possibility of more resignations in the coming days and weeks, sources say. The Minneapolis U.S. Attorney's office typically has between 60 and 70 attorneys.