Arpaio on Thursday unveiled preliminary results of an investigation, conducted by members of his volunteer cold-case posse, into the authenticity of President Barack Obama's birth certificate, a controversy that has been widely debunked but which remains alive in the eyes of some conservatives.

At a news conference, Arpaio said the probe revealed that there was probable cause to believe Obama's long-form birth certificate released by the White House in April is a computer-generated forgery. He also said the selective service card completed by Obama in 1980 in Hawaii also was most likely a forgery.

"We don't know who the perpetrators are of these documents," Arpaio said, although he said he doesn't think the president forged the documents.

Earlier, the 79-year-old Republican sheriff defended his need to spearhead such an investigation after nearly 250 people connected to an Arizona tea party group requested one last summer.

"I'm not going after Obama," said Arpaio, who has criticized the president's administration for cutting off his federal immigration powers and conducting a civil rights investigation of his office. "I'm just doing my job."