The Arizona probe is one of 20 similar federal investigations under way nationwide by Justice's Civil Rights Division - "more than any time in the division's history," Assistant Attorney General Thomas E. Perez said.
Perez, in a letter of warning Thursday to Maricopa County officials, said that "deputies, detention officers, supervisory staff and command staff, including Sheriff Arpaio, have engaged in a widespread pattern or practice of law enforcement and jail activities that discriminate against Latinos."
He described Latinos arrested on unreasonable traffic stops, businesses raided when Latinos gather out front, inmates mocked with racial epithets, and 432 cases of sexual assault and child molestation, often involving Latinos, that "were not properly investigated over a three-year period."
One Latino was intentionally hit by a patrol car and dragged, with instructions for other deputies to "leave him there," prosecutors said. A Latino motorist was incarcerated for 13 days for not using his turn signal. E-mails written by deputies caricatured Mexicans as being from "Mexifornia," and deputies derided Latino inmates as "wetbacks," "Mexican bitches," "stupid Mexicans," and unprintable epithets.
Prosecutors said the abuse begins with deputies targeting Latino drivers, who it said were four to nine times more likely than whites to be stopped, and said officers "treat Latinos as if they are all undocumented, regardless of whether a legitimate factual basis exists to suspect that a person is undocumented."
The findings set the stage for a faceoff with the often-abrasive sheriff.
Perez warned that if Arpaio was not interested in making drastic changes, "we are prepared to file a civil action to compel compliance." A federal criminal investigation of the department's public corruption unit is continuing.
Arpaio said at a televised news conference in Arizona that he would try to cooperate but that "if they are not happy, I guess they can carry out their threat and go to federal court."
He criticized the Justice Department findings as "a sad day for America as a whole," and said that federal intervention into his sheriff's office would only lead to the release of jail inmates being held on immigration charges after committing previous offenses.
"Don't come here and use me as a whipping boy for a national and international problem," he said. "We are proud of the work we have done to fight illegal immigration."
Arpaio, 79, was first elected sheriff in 1992 and has remained a popular figure, particularly among conservative illegal-immigration hawks.
Maricopa County, which has nearly four million residents, has been particularly troubled by illegal immigration. The area has become a hotbed of gun running, kidnappings, and drug smuggling.
Arpaio has responded by housing inmates in tents, clothing them in pink underwear, and serving green and blue meat.
Rep. Raul M. Grijalva (D., Ariz.) said the sheriff's department could not be reformed unless Arpaio stepped down.
"He's the one who made himself a national figure by violating people's rights," Grijalva said.