Image caption Joe Arpaio has served five terms as a hard-nosed sheriff and has been closely involved in immigration issues

The birth certificate of US President Barack Obama could be a forgery, a controversial Arizona sheriff claims.

Sheriff Joe Arpaio of Maricopa County - who styles himself as America's toughest - unveiled the results of a probe into the origins of the document.

His investigators found "probable cause" the certificate may have been made by "forgery and fraud", he said.

Mr Arpaio, known for his tough stance on immigration, is being investigated over allegations of racial profiling.

The US justice department alleges that his office routinely discriminated against Latinos.

He also faces a federal grand jury investigation into the activities of his anti-corruption unit, and a forthcoming re-election bid in Maricopa County.

Tea Party request

Announcing the findings of his volunteer "Cold Case Posse", Mr Arpaio, 79, said analysis of an electronic copy of Mr Obama's birth certificate had revealed strong doubts about its authenticity.

"Based on all of the evidence presented and investigated I cannot in good faith report to you that these documents are authentic," Mr Arpaio said.

If nothing else comes out of the investigation… what we have learned, we need a better process to vet people running for president of the United States of America Joe Arpaio, Maricopa County sheriff

"My investigators believe that the long-form birth certificate was manufactured electronically and that it did not originate in a paper format as claimed by the White House."

The White House released Mr Obama's long-form birth certificate in April 2011 amid suggestions by conspiracy theorists that he might not be a natural born US citizen.

That would make Mr Obama, whose father is Kenyan, ineligible to hold the office of president.

The release of the birth certificate appeared to placate some so-called "birthers", among them businessman and reality TV star Donald Trump, who had publicly called for greater clarity.

But Mr Arpaio said he had begun an investigation during 2011 after requests from Arizona Tea Party members.

According to the sheriff, the investigators compared the electronic copy of Mr Obama's birth certificate with one known to be genuine.

The conclusion that "probable cause" existed warranted a criminal investigation and greater scrutiny of presidential candidates, Mr Arpaio said.

"If nothing else comes out of the investigation… what we have learned, we need a better process to vet people running for president of the United States of America," he said.

Critics have suggested that Mr Arpaio launched his probe into Mr Obama's birth certificate to distract attention from his legal wrangles, as well as to aid his upcoming re-election bid.

But the sheriff denied those accusations, telling reporters at his news conference on Thursday: "I'm not going after Obama. I'm just doing my job."