The outspoken sheriff of Maricopa County is not one to buckle under pressure. About two months after the US Justice Department accused him of racial discrimination, Joe Arpaio is back in the spotlight, dousing on-again, off-again controversy over President Obama's place of birth.

America's self-proclaimed toughest sheriff called a news conference Thursday to report that members of his volunteer cold-case posse found probable cause to believe that the long-form birth certificate the White House released last April is a computer-generated forgery. They found the same about his selective-service card from 1980.

“I cannot in good faith report to you that these documents are authentic,” said the Republican lawman, whose unyielding stance against illegal immigration propelled him to national prominence.

Rumors that Mr. Obama was born outside the United States, in his father's homeland of Kenya, have been debunked time and again. But some refuse to believe the president was born in Hawaii, as his certificate shows, and contend he is ineligible to hold the nation's highest elected post. Arpaio says he launched the probe at the request last summer of 250 Arizonans with ties to the “tea party.”

Critics say Arpaio has much to gain by injecting himself into the so-called birther issue, which could deflect attention from his troubles while appealing to conservative supporters. He remains under federal investigation for potential civil rights violations, and a separate federal probe centers on accusations of abuse of power.

As a result, his ability to enforce immigration laws has been curtailed. And several groups have called for his resignation over news reports that his office has neglected hundreds of sex-crimes cases.

Given Arpaio's clashes with the Obama administration, his latest behavior “becomes kind of a standoff with the government,” says Bruce Merrill, a political scientist and professor emeritus at Arizona State University in Tempe.

“He retaliates against his critics,” adds Randy Parraz, co-founder and president of Citizens for a Better Arizona and one of Arpaio's harshest critics.

Arpaio denies his motivations are political in nature and insists that he is just doing his job.

“It's just like any other criminal investigation,” he says.

Mr. Merrill doubts the legitimacy of Arpaio's findings. “He's a sheriff; he's not a trained forensic scientist,” he says.

The sheriff may be trying to drum up support as he seeks to be elected to a sixth term this year, says Paul Bender, a law professor at Arizona State.

“He likes to be in the public eye,” he says. “He keeps doing things to keep himself that way, and it's worked for him and his political career so far because he's still in office.”

The professor and others maintain that the state's tough immigration law, known as SB 1070, is unconstitutional. But Arpaio is a staunch advocate of the law, which is tied up in court.

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The sheriff's zeal for chasing illegal immigrants has made him a favorite among like-minded people, such as state Rep. Carl Seel, who plans to revive efforts to pass birther legislation in Arizona. But the sheriff's actions also have brought scrutiny. In September 2010, the Justice Department filed a lawsuit against the sheriff, saying he had refused to cooperate with a civil rights investigation. Then, in mid-December of last year, the Justice Department after a three-year investigation issued a scathing report against his office over consistent bias against Latinos and retaliation against those who complained. The sheriff maintains that the federal government's actions are politically motivated.

Despite his predicaments, Arpaio is still a player in Republican circles. The GOP presidential candidates for months sought after his endorsement while campaigning in Arizona. Arpaio endorsed Rick Perry, who eventually dropped out of the race.