A federal investigation of a controversial Arizona sheriff known for tough immigration enforcement has intensified in recent days, escalating the conflict between the Obama administration and officials in the border state.

Justice Department officials in Washington have issued a rare threat to sue Maricopa County Sheriff Joe Arpaio if he does not cooperate with their investigation of whether he discriminates against Hispanics. The civil rights inquiry is one of two that target the man who calls himself "America's toughest sheriff." A federal grand jury in Phoenix is examining whether Arpaio has used his power to investigate and intimidate political opponents and whether his office misappropriated government money, sources said.

The standoff comes just weeks after the Justice Department sued Arizona and Gov. Jan Brewer (R) because of the state's new immigration law, heightening tensions over the issue ahead of November's midterm elections. The renewed debate has focused attention on Arpaio, a former D.C. police officer who runs a 3,800-employee department, and a state at the epicenter of the controversy over the nation's estimated 12 million illegal immigrants.

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Once seen as a quirky figure who has inmates dress in pink underwear and forces them to work on chain gangs, Arpaio has in recent years become a kind of folk hero to those who favor his heavily publicized "crime sweeps," conducted mostly in Hispanic neighborhoods. But civil rights groups accuse the 78-year-old lawman of racial profiling. And some Maricopa County officials say Arpaio has begun meritless corruption investigations of officials who have criticized his policies or opposed his requests.

Those allegations are at the core of the Justice Department investigations, according to documents, lawyers familiar with the inquiries, and people who have been questioned by FBI agents and the grand jury.

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The investigations reflect the tangled politics surrounding the immigration debate. The criminal probe is led by Dennis K. Burke, the U.S. attorney in Phoenix who was a top aide to Homeland Security Secretary Janet Napolitano.

Two of Arpaio's attorneys, Robert N. Driscoll and Asheesh Agarwal, were officials in the Justice Department's civil rights division in the George W. Bush administration. They denied that the sheriff, a Republican who has been reelected four times since 1992, has been uncooperative or has engaged in racial profiling, misusing money or targeting political enemies.

"The sheriff's office is cooperating fully with the grand jury investigation and has complete confidence that the inquiry will clear it of any wrongdoing," Agarwal said. "The office has always fulfilled its responsibilities truthfully, honorably, and in full compliance with state and federal law."

Arpaio's attorneys contend that the investigations are politically motivated, citing a news conference in March at which Attorney General Eric H. Holder Jr. was quoted as saying he expects the inquiries to "produce results."

"While we have no quarrel with the assistant U.S. attorneys handling the investigation, the attorney general's comments appear to violate federal regulations, departmental policy and state ethical rules designed to ensure the fairness of criminal investigations," Agarwal said.