The U.S. Justice Department has launched a civil-rights investigation of the Maricopa County Sheriff's Office after months of mounting complaints that deputies are discriminating in their enforcement of federal immigration laws.

Officials from the Justice Department's Civil Rights Division notified Sheriff Joe Arpaio on Tuesday that they had begun the investigation, which will focus on whether deputies are engaging in "patterns or practices of discriminatory police practices and unconstitutional searches and seizures."

An expert said it is the department's first civil-rights probe related to immigration enforcement.

Arpaio vehemently denies that deputies are illegally profiling as part of his immigration crackdowns. He said Tuesday that he welcomes the investigation and intends to cooperate fully.

"We have nothing to hide," he said.

Although Arpaio's illegal-immigration crackdowns have broad public support, they also have led to calls for an examination of his tactics.

Last year, Phoenix Mayor Phil Gordon asked for a federal investigation of possible civil-rights abuses. Last month, four key Democratic members of the U.S. House Judiciary Committee asked Attorney General Eric Holder and Homeland Security Secretary Janet Napolitano to investigate Arpaio.

The lawmakers said Arpaio had exceeded the limits of a federal program that gives local police federal immigration-enforcement powers by ordering deputies to "scour" Latino neighborhoods looking for illegal immigrants based on skin color.

Arpaio, who was easily re-elected to a fifth term in November, called the investigation politically motivated and vowed to continue to arrest illegal immigrants.

"I am not going to be intimidated by the politics and by the Justice Department," Arpaio said. "I want the people of Arizona to know this: I will continue to enforce all the immigration laws."

Arpaio uses the sweeps to enforce the state's employer-sanctions and anti-smuggling laws. He also participates in a federal program that lets local officers enforce federal immigration laws. The sweeps have taken place in mostly Latino neighborhoods or near where day laborers congregate. They have sparked two racial-profiling lawsuits.

The Justice Department frequently receives racial-profiling complaints against police departments, but investigations are rare, said David Harris, a University of Pittsburgh law professor and racial-profiling expert.

"The fact that this has come to their attention and they have announced their intent to investigate is highly significant," Harris said. "It says there is enough there to be investigated. It's not an iffy case that (can be ignored)."

Harris said this is the first civil-rights investigation stemming from immigration enforcement. The probe could last several months.

In a two-page letter dated Tuesday, Loretta King, acting assistant attorney general, said that if the investigation uncovers violations, her office will work with Arpaio to find remedies.

But Arpaio said he will battle the Justice Department in court if he disagrees with any of the changes the department tries to impose.

In the 1990s, the department conducted similar civil-rights investigations and found patterns of police discrimination in about 20 cases, including in Los Angeles and Pittsburgh. In those cases, law-enforcement agencies agreed to significant changes aimed at preventing discrimination or face a court injunction, Harris said.

"Once the Justice Department finds violations, the threat of going to court is usually enough to encourage them to agree to change," Harris said. Changes have included increased supervision and changing policies, Harris said.

Investigations into patterns of police discrimination are "not about punishing individual officers; they are about changing the fundamental" way an agency operates, he said.

Gordon, who met with King and Deputy Assistant Attorney General John Wodatch on Tuesday in Washington, praised the investigation.

"We should all be encouraged that our new attorney general is taking these issues seriously," he said.

Maricopa County Supervisor Mary Rose Wilcox, the board's lone Democrat and most vocal critic of Arpaio's immigration policies, had planned to help deliver a petition today with 35,000 Internet signatures calling for a Justice Department investigation.

"I think they're going to find racial profiling, which is a civil-rights abuse," said Wilcox, who was in Washington for a National Association of Counties conference. "It's time to put a stop to them. It may cost us millions in lawsuits."

Board Chairman Max Wilson, one of the board's four Republicans, said he was surprised by the investigation.

"I know there's been some accusations made," he said. "I don't know if there's any merits to them. I've almost had my hands full of people making accusations without people having some solid, hard evidence to back it up."

Republic reporters Dennis Wagner, Yvonne Wingett and Scott Wong contributed to this article.