Assistant U.S. Attorney Ann Birmingham Scheel released a statement saying her office "is closing its investigation into allegations of criminal conduct" by current and former members of the sheriff's and county attorney's offices.
Scheel, who is based in Arizona, said she was acting on behalf of the U.S. Department of Justice. In a four-page letter to Maricopa County Attorney Bill Montgomery explaining the decision, Scheel wrote that "our limited role is to determine whether criminal charges are supportable. After careful review, we do not believe the allegations presented to us are prosecutable as crimes."
"I've been in law enforcement for 50 years. Nothing surprises me. But I know my people did the right thing," said Arpaio, 80, who is running for a sixth term as sheriff in Arizona's most populous county. "I'm just happy for my organization, for my deputies. Not for me."
The federal probe focused specifically on the sheriff's anti-public-corruption squad. In a separate probe, the Justice Department has accused Arpaio's office of a wide range of civil rights violations, and in another case, a federal judge has yet to rule in a civil case brought by a group of Latino plaintiffs that alleged Arpaio and his deputies engaged in racial profiling.
The timing of the federal authorities' announcement - at 5 p.m. on a Friday before a holiday weekend - was questioned by some Arpaio critics.
"It is a miscarriage of justice that the federal government is dropping its case against Sheriff Arpaio and to make such an announcement on the Friday night before the Democratic National Convention can only be politically motivated to shield the administration from criticism," Pablo Alvarado, director of the National Day Laborer Organizing Network, said in a statement.
Arpaio and his top ally, former County Attorney Andrew Thomas, were embroiled in a three-year feud with county officials and judges and defended their investigations as necessary to root out corruption.
The officials who were targets of the investigations contend the probes were trumped up as retaliation for political and legal disagreements with the sheriff and prosecutor.