The Arizona lawman's No. 2 man and other top officials are accused of using the anti-corruption unit to conduct politically motivated investigations and to surveil campaign rivals.

Munnell also said that Hendershott asked two other sheriff's officials to surveil Saban's meetings in 2008 to see which department employees were supporting him. They refused. In 2000, Munnell writes, Hendershott directed several sheriff's officials to use a volunteer's vehicle to watch a meeting held on behalf of Arpaio's then-challenger to identify department employees who were supporting him.

Arpaio has said that his anti-corruption investigations were legitimate and that ongoing federal investigations into his department are politically motivated. Earlier this month, the Department of Justice sued his office, alleging that Arpaio was refusing to produce basic paperwork in the civil rights probe.

"I find it very unsettling that this office stonewalls all investigations targeting this office, claiming they are political," Munnell wrote. "However, when this office investigates public officials, we have the audacity to publicly criticize their failure to cooperate with our investigators."