El Mirage, a heavily Hispanic community near Phoenix, alleged there were many cases in which sheriff's investigators wrote no follow-up reports, collected no additional forensic evidence and made no effort after the initial crime report was taken.
Arpaio's office eventually reopened more than 400 of its sex-crime cases countywide after finding they were inadequately investigated or not examined at all. The botched investigations have been an embarrassment to a department whose sheriff is the self-described "America's toughest sheriff" and a national hero to conservatives on immigration issues.
Arpaio apologized in December 2011 for the bungled cases, and his office has since said it has moved to clear up the cases and taken steps to prevent the problem from happening again.
The internal investigation launched in May 2008 was stopped after its investigator was pulled away at the direction of David Hendershott, Arpaio's then-top aide, to help with another matter. The probe was reopened in December 2010 while Hendershott was on medical leave.
The botched investigations were mentioned in a lawsuit by the U.S. Justice Department that alleges a range of civil rights violations in Arpaio's immigration patrols and jails.
The Justice Department accused the sheriff's office of failing to adequately respond to reports of sexual violence and focusing intensively on low-level immigration offenses over more serious crime. Arpaio's office has denied the allegations.
Arpaio's critics used the bungled investigations to hammer on the sheriff last year as he was campaigning for a sixth term. He was forced to plow millions of dollars into the race to fend off the challenge. In the end, Arpaio won by a six-point margin.
Last month, a group launched a campaign to call a recall election against the six-term sheriff.