© Associated Press/Ross D. Franklin
Maricopa County Sheriff Joe Arpaio
US - Armed with a new 10-page report he commissioned, Maricopa County Sheriff Joe Arpaio said Thursday he suspects the birth certificate President Obama released last year is a "computer-generated forgery" - and also raised questions about the authenticity of the president's selective service registration card.

In an extensive - and at times combative - press conference in Phoenix, Sheriff Arpaio repeatedly said he is not questioning the president's legal status under the Constitution nor alleging fraud on Mr. Obama's part, but did say there is evidence crimes have been committed by someone and his investigation continues.

"Based on all of the evidence presented and investigated, I cannot in good faith report to you that these documents are authentic," Sheriff Arpaio said. "My investigators believe that the long-form birth certificate was manufactured electronically and that it did not originate in paper format as claimed by the White House."

He said he would like Congress to investigate the matter, but short of that, he has an obligation to look into it because of potential violations of Arizona's laws.

The White House did not respond to a request for comment Thursday.

Questions about Mr. Obama's birth certificate have been a political sideshow for the past four years, and have persisted despite repeated denials by the White House and the release of evidence he was born in Honolulu on Aug. 4, 1961. That evidence included both a certificate of live birth released during the 2008 campaign and the long-form certificate last year.

Fact-checking organizations have concluded the certificates are authentic, and Mr. Obama has joked about those who questioned his birth. At last year's White House Correspondents Association Dinner, soon after releasing his long-form certificate, he mocked Donald Trump, who had said his own investigators were looking into the matter.

But Sheriff Arpaio, who first built his national profile by creating tent cities for jail inmates and later by taking a tough stance on illegal immigration, could reignite questions among those who continue to believe Mr. Obama was born outside the U.S. and is ineligible to be president under the Constitution's provision that the officeholder be a natural-born citizen.

Last December, the Justice Department accused Sheriff Arpaio of civil rights abuses within his department, but he said he began his investigation before that - in August 2011 - after tea party members from Surprise, a town within Maricopa County, signed a petition challenging the certificate.

Activists protested outside of the auditorium where Sheriff Arpaio held his press conference, accusing him of running a corrupt department. A few miles away, outside the Phoenix main library branch, a man was collecting signatures to back Lt. Mike Stauffer, a Scottsdale police officer and a Republican who is challenging Sheriff Arpaio in the upcoming election.

"We hate him here," the man gathering signatures said of Sheriff Arpaio.

Sheriff Arpaio said he went into the investigation with an open mind, and would have been happy to clear Mr. Obama.

Instead, he said the former law enforcement investigators he deployed from his "cold case posse" - at no taxpayer expense, he said - were unable to back up the White House's story.

His investigators said the image of the birth certificate released by the White House appears to have been created originally on a computer, rather than having been originally made on paper and then scanned.

The investigators said they have identified "a person of interest" in the birth certificate matter.

Sheriff Arpaio's chief investigator on the project said there are so many questions that he couldn't have cleared Mr. Obama to be an employee of Maricopa County.

Pursuing questions about the birth certificate has been a thorny issue for the GOP. State lawmakers in several places, including Arizona, have pursued laws that would have imposed checks on any presidential candidate's qualifications to get on their state ballots.

One such proposal in Arizona, which became known as the "birther" bill, was vetoed by Gov. Jan Brewer last year.