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By doing so, the Republican joined a fringe group of activists and writers who believe in a conspiracy theory that has been debunked several times over by numerous independent investigations.
"It's not over," the sheriff said during the hour-long event. "This investigation is not over."
Arpaio's belief puts him at odds with some of the same Republican presidential candidates who have courted his endorsement in recent months. It also drew quick fire from one of his fellow members of the Arizona GOP.
"Sheriff Joe reveals results of his Obama birth 'investigation' today," former state Attorney General Grant Woods wrote on Twitter. "To say he has become an embarrassment is an understatement."
A Democrat hoping to unseat Arpaio in November blasted the sheriff in a written statement afterward.
"Today's news conference by the sheriff is an insult to law enforcement and a slap in the face to every victim his office has failed to serve," said former Phoenix police officer Paul Penzone. "Maricopa County deserves better, Arizona deserves better."
The campaigns of Republican presidential contenders Mitt Romney, Newt Gingrich and Rick Santorum did not immediately respond to TPM's questions about whether they would still take Arpaio's endorsement if he offered it.
The candidates have gladly accepted endorsements from conspiracy theorists in the past. Notably, Romney has campaigned with flamboyant real estate mogul Donald Trump, who continues to express skepticism of Obama's birth certificate.
Birther proponents generally believe Obama was not born in the United States and therefore is ineligible to be president.
Arpaio told reporters that his investigation into Obama's birth certificate turned up "probable cause" to show that two crimes had been committed regarding the birth certificate — fraud and forgery. An investigator who spoke at the event said they have identified a "person of interest" in the supposed crime, but he declined to say who it was.
"I am not accusing the president of the United States of any crime," Arpaio said. "We're going to find out who did it."
Arpaio said he recognized that his new stance would draw criticism, something the sheriff has reveled in throughout his career.
"If I'm being criticized for enforcing the law, on felonies, then something is wrong," he said. "Those responsible, whoever they are, should be brought to justice."
Arpaio was joined at the news conference by Jerome Corsi, a writer for the conservative website WorldNetDaily and one of the main proponents of the birther theory. Corsi said he assisted in the sheriff's investigation, which has been going on for six months and was run through Arpaio's "Cold Case Posse." The sheriff said the investigation was funded completely by donations from private citizens.
The bulk of the evidence that Arpaio's investigators presented to the reporters seemed to come from theories that have been thoroughly debunked by other sources, including the conservative National Review Online.
The focus of much of the presentation centered on "white halos" surrounding the letters on Obama's birth certificate.
Arpaio dodged questions about whether he was holding the event to distract from the fact that he is the target of civil and criminal investigations by the Justice Department.
"Look at the documents. Stick with that. Not all these other politics," he said. "I like to think that we're going to settle this for once and for all."
TPM reporters Evan McMorris-Santoro and David Taintor contributed to this report.