Arpaio has said he took deliberate steps to avoid the appearance that his investigation is politically motivated. Instead of using taxpayer money, the sheriff farmed it out to lawyers and retired police officers who are volunteers in a posse that examines cold cases. Other posses assist deputies in duties that include providing free police protection at malls during the holiday season or transporting people to jail.
Even as he is under fire by the federal government, the sheriff remains popular among Republicans.
GOP presidential candidates have courted him for his endorsement throughout the primary season. At last week's GOP presidential debate in Arizona, Arpaio won loud cheers. During a question about Arizona's border woes, Pennsylvania Sen. Rick Santorum said the government ought to give local police agencies the chance to enforce immigration law as Arpaio has.
Bruce Merrill, a longtime pollster and senior research fellow at Arizona State University's Morrison Institute for Public Policy, said the subject of the investigation plays to the sheriff's base of supporters. And, he said, it highlights Arpaio's gift for publicity.
"It's something that the press will cover," Merrill said. "He'll get a lot of exposure from it."