PHOENIX —  The Justice Department sued the nation’s self-proclaimed “toughest sheriff” on Thursday, calling Joe Arpaio’s defiance of an investigation into his office’s alleged discrimination against Hispanics “unprecedented.”

It’s the first time in decades a lawman has refused to cooperate in one of the agency’s probes, the department said.

The Arizona sheriff had been given until Aug. 17 to hand over documents the federal government first asked for 15 months ago, when it started investigating alleged discrimination, unconstitutional searches and seizures, and jail policies that discriminate against people with limited English skills.

Thomas Perez, assistant attorney general for the department’s civil rights division, said it’s unfortunate the department had to sue to get the documents, which neither the agency nor Arpaio would describe.

But Arpaio called the lawsuit “a ruse” and said the federal government is just trying to score a win against the state, which has found itself at the center of the nation’s argument over illegal immigration since passing a law that mirrors many of the policies Arpaio has put into place in the greater Phoenix area.

“I think they know we have not been racial profiling, so what’s the next step — camouflage the situation, go the courts, and make it look like I’m not cooperating,” Arpaio said Thursday.

Arpaio said he provided “hundreds of thousands” of reports but hasn’t turned over others because the department’s request was too broad.

Kevin Ryan, former U.S. attorney for the Northern District of California and a law professor at the University of San Francisco, said he thought the department’s characterization of Arpaio’s behavior as unprecedented was overstating it.

He said the contentious relationship between the sheriff and the department is no secret.

“You really can’t hold it against the sheriff and assume he’s guilty because he’s not rolling over for the Justice Department,” he said.

But Rory Little, a law professor at the University of California Hastings College of the Law who formerly worked at the Justice Department, disagreed, called Arpaio’s actions “pretty unusual” because the lawsuit says Arpaio’s office signed agreements promising to cooperate with civil-rights investigations and other reviews when it accepted federal law enforcement grants.

Last year, the nearly $113 million that Maricopa County received from the federal government accounted for about 5 percent of the county’s $2 billion budget. The lawsuit listed $16.5 million of funding provided Arpaio’s office through several programs.

“Normally when you receive $113 million in grants you’re going to cooperate and send over whatever they want to see,” Little said. Otherwise, “it raises the level of suspicion pretty significantly.”

He also said it’s rare for a law enforcement agency to push the department all the way to a lawsuit.

“Cooperating with the Department of Justice is usually not a bad thing so long as you’re not the target of a criminal investigation, and the federal government has a lot of power in terms of grants and you don’t want to get on their bad side,” he said.

Arpaio believes the department’s inquiry is focused on his immigration sweeps, patrols where deputies flood an area of a city — in some cases heavily Latino areas — to seek out traffic violators and arrest other offenders.

Critics say the deputies pull people over for minor traffic infractions because of the color of their skin so they can ask them for their proof of citizenship.

Thursday’s lawsuit is the latest action in a slew against Arizona by the federal government.

In 2009, the U.S. Department of Homeland Security stripped Arpaio’s office of its special powers to enforce federal immigration laws, and in May, the Obama administration urged the U.S. Supreme Court to prevent Arizona from enforcing its employer sanctions law.

In July, the Justice Department filed a lawsuit to overturn portions of Arizona’s strict new immigration law that would require police officers to question people about their immigration status if there is reason to suspect they are in the country illegally. A federal judge put that provision and most of the law on hold.

The continued attention on the state sends a clear political message that the federal government doesn’t want Arizona enforcing federal immigration laws, said Mark Krikorian, executive director of the Washington, D.C.-based Center for Immigration Studies, which advocates for strict immigration laws.

“It’s surprising that the administration would focus on Arizona and go after it on such a high-profile and persistent way,” he said.

In a separate investigation, a federal grand jury in Phoenix is examining allegations that Arpaio has abused his powers with actions such as intimidating county workers by showing up at their homes at nights and on weekends.

A Hispanic activist said a federal judge might have to threaten jail time to get Arpaio to cooperate in the lawsuit filed Thursday.

“It’s going to take the hard hand of the judge to order some sanctions against the sheriff’s office,” said Lydia Guzman of the Phoenix-based civil rights group Somos America.

Arizona Republican Sen. Russell Pearce, author of the new Arizona law, called the Justice Department’s actions against Arpaio a “witch hunt.”

“This is the game that’s played,” he said. “They couldn’t find any violations ... that’s why they’re very vague about what they want. It doesn’t take a very high IQ to figure out what’s going on with these folks.”

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Associated Press Writer Jacques Billeaud contributed to this report.