Arpaio said the deputy is working on other cases from Hawaii, where he has joined a volunteer with the Maricopa County cold-case squad, The Arizona Republic reported.
"It's one deputy, so what? We have security issues, too, that I can't got into," Arpaio said. "For six months we were not spending any money. When you're doing investigations, sometimes things change. You put more resources into it."
Arpaio, sheriff in Maricopa County, which includes Phoenix and surrounding suburbs, said the cold-case posse, a group of volunteers operating with donations, is paying for most of the investigation. He said he began the investigation last year when 250 members of the Tea Party in Surprise, Ariz., requested it.
He said the investigation is focused not on Obama's birthplace but on whether his birth certificate is legitimate.
Obama's Hawaiian birth is documented by his birth certificate and by a 1961 birth announcement in a Honolulu newspaper that would have gotten its information from public records, not from his family.
Arpaio denies his investigation is a response to Justice Department allegations that his department discriminates against Hispanics.
Arizona Secretary of State Ken Bennett -- who had hinted at the possibility Obama would not be on the state's presidential ballot in November -- said Tuesday he expects the president will be on the ballot. Bennett, the head of presumptive Republican nominee Mitt Romney's Arizona campaign, told KTAR-FM, Glendale, Ariz., he believes Obama was born in Hawaii, as Hawaiian official records confirm.
"I'm not a birther, which means I do not subscribe to the opinion that he was born somewhere else," Bennett said.
"If I embarrassed the state, I apologize," Bennett said.