Sheriff Joe Arpaio's volunteer investigation into documents pertaining to President Barack Obama's place of birth and citizenship now includes the services of a taxpayer-funded deputy, according to Arpaio.

The deputy joined Mike Zullo, a volunteer member of Arpaio's cold-case posse, in Hawaii this week in part due to "security issues," according to Arpaio, and because the investigation has progressed.

"It's one deputy, so what? We have security issues, too, that I can't get into," Arpaio said on Friday. "For six months, we were not spending any money. When you're doing investigations, sometimes things change, you put more resources into it."

The cold-case posse has spent about $40,000 on the investigation so far, according to the Sheriff's Office. The posse is funded through donations.

Arpaio has in the past touted the investigation's total reliance on donated funds and volunteers, but he said the investigation now requires the use of a sworn deputy. Arpaio said the volunteer posse is in the midst of a criminal investigation but refused to discuss details of the probe or what drove the posse member and deputy to Hawaii.

The Sheriff's Office also had to cover the costs of airfare and hotel rooms for the deputy, Arpaio said, but he expects the posse to reimburse the agency for those costs.

The detective assigned to the Obama investigation works in the sheriff's threats unit and continues to work on other cases while assisting with the posse's investigation, according to the Sheriff's Office.

"He's not going to make any arrests," Arpaio said. "I didn't say we're going to keep using him. We're not going to use him constantly. He's not assigned to it. For this trip, I feel it's important to have a deputy there. He's just a liaison to give advice if needed. He's not doing anything. The posse's been doing the research. I'm not going to say what other trips they've been taking, but they haven't had a deputy with them."

Arpaio is not the only Arizona official with an election-year interest in discussing records related to Obama's birth.

Arizona Secretary of State Ken Bennett recently asked for confirmation of President Obama's birth as he worked to confirm the President is illegible to be on the Arizona ballot. Bennett recieved verification Tuesday night and said the matter is closed.

The sheriff's investigation was launched last year, Arpaio said, at the request of 250 members of the Surprise Tea Party.

In March, the sheriff held a news conference with Zullo; author Jerry Corsi, whose research initially guided the investigation; and members of the Surprise Tea Party. Zullo detailed the posse's allegations that Obama's birth certificate and Selective Service card were fraudulent.

The White House pointed to documents available on the government's website that verify the president's birth in Hawaii. Copies of those documents were what Zullo relied on in the investigation.

Earlier this month, the U.S. Justice Department filed a civil-rights lawsuit alleging that the Sheriff's Office has discriminated against Latinos in jail and patrol operations and that his office has retaliated against people who speak out against Arpaio and his policies.

Arpaio has denied that the Obama investigation is politically motivated, though critics have pointed out that other skeptics and many mainstream Republicans long ago abandoned the topic.

Arpaio said his investigation isn't focused on Obama's birthplace, but rather the potential that some of the documents produced to verify his birth are fraudulent.

"I have enough deputies around to spare one," Arpaio said. "One deputy out of 900 and we're going to have problems? No way."

Republic reporter Yvonne Wingett Sanchez contributed to this article.