PHOENIX - The U.S. Justice Department sued America's self-proclaimed toughest sheriff Thursday, a rare step for the agency after months of negotiations failed to reach a settlement over allegations that his department racially profiled Latinos in his immigration patrols.

Federal officials said that only once before had the agency filed a lawsuit against a police department that they were unable to reach an agreement with in the 18-year history of the DOJ's police reform efforts. The lawsuit means that a federal judge will decide the escalating standoff with Sheriff Joe Arpaio.

"We have invariably been able to work collaboratively with law enforcement agencies to build better departments and safer communities," said Assistant U.S. Attorney General Thomas Perez, who heads the DOJ's civil rights division, at a news conference.

Arpaio and the Maricopa County Sheriff's Office "have been a glaring exception," Perez said.

The DOJ first leveled the allegations against Arpaio in December, saying that a culture of disregard for basic constitutional rights prevailed at the office, which covers metro Phoenix. Talks to reach a settlement broke off last month.

At the time, Arpaio refused to agree to a court-appointed monitor who would help enforce a settlement. He said it would mean every policy decision would have to be cleared through an observer and would nullify his authority.

Arpaio, who planned to hold a news conference Thursday to discuss the lawsuit, issued a brief statement.

"The Department of Justice would like to take over my office by requiring the implementation of a court-appointed monitor prior to ever producing any evidence that there has been a systemic problem with discrimination by my office," Arpaio said. "So far, their claims are based on isolated incidents that have already been identified and addressed."

A federal law passed in the aftermath of the Rodney King verdict and the Los Angeles riots prohibited police agencies from systematically violating constitutional rights. Normally, settlements are negotiated and filed in court as part of lawsuits that are not contested by the police agencies.

In enforcing the law, the DOJ has taken only the Columbus, Ohio, Police Department to federal court, in 1999, but the police agency eventually settled the case, Perez said.

In addition to racial profiling, Arpaio's office is accused of punishing Hispanic inmates for speaking Spanish and launching some patrols based on complaints about dark-skinned people congregating in a given area or speaking Spanish. A crime was never reported.

The sheriff has said the investigation was a politically motivated attack by the Obama administration, denied allegations of systematic discriminatory policing, and insisted that the Justice Department provide facts to prove its allegations. The Justice Department has said a 22-page letter it sent to Arpaio in December provided those details.