Under the leadership of the flamboyant Joe Arpaio, the sheriff's department of Maricopa County, Arizona, "engages in racial profiling of Latinos," the U.S. Justice Department says in a report released Thursday.

The Maricopa County Sheriff's Office also retaliates against critics and discriminates against Latino inmates in detention, according to the findings of a probe that began in June 2008.

"(W)e found reasonable cause to believe that MCSO engages in a pattern or practice of violating the Constitution and laws of the United States," Assistant Attorney General Thomas E. Perez said at a press conference in Phoenix, the Maricopa County seat.

"As with all of our investigations, our mission here was, and will continue to be, to determine the truth," the head of the Justice Department's Civil Rights Division said. "We did not begin this investigation with any preconceived notions."

Federal investigators interviewed more than 400 people, including Arpaio, and reviewed thousands of documents.

"Latino drivers were four to nine times more likely to be stopped than similarly situated non-Latino drivers," an expert on racial profiling concluded after a statistical analysis of MCSO traffic stops, Perez said.

The Justice Department also found that critics of the MCSO's policies "were frequently arrested and jailed for no reason, or forced to defend against specious civil complaints or other baseless charges."

Perez said evidence of discrimination against Spanish-speaking inmates in detention facilities is "particularly troubling because MCSO has been on notice for years of problems in the jail."

While a 2010 MCSO policy memo "acknowledged the importance of being able to communicate with Spanish-speaking inmates in Spanish," the department failed to follow through, the assistant attorney general said.

"We are not talking about isolated incidents. We found discriminatory policing that was deeply rooted in the culture of the department," Perez said.

The 79-year-old Arpaio, who boasts of being "the toughest sheriff in the West," has devoted enormous resources to pursuing undocumented immigrants.

The Justice Department gave the MCSO a deadline of Jan. 4 to accept a process of voluntary cooperation with the federal government to remedy the problems cited in the report.

If Arpaio refuses to work with Washington, the department could sue the MCSO to compel compliance.