Members of Maricopa County Sheriff Joe Arpaio's posse said in March that there was probable cause that Obama's long-form birth certificate released by the White House in April 2011 was a computer-generated forgery.
Now, Arpaio says investigators are positive it's fraudulent.
Mike Zullo, the posse's chief investigator, said numeric codes on certain parts of the birth certificate indicate that those parts weren't filled out, yet those sections asking for the race of Obama's father and his field of work or study were completed.
Zullo said investigators previously didn't know the meaning of codes but that the codes were explained by a 95-year-old former state worker who signed the president's birth certificate. Zullo said a news reporter who has helped out in the probe let investigators listen in on an interview he concluded of the former state worker.
The Obama campaign declined to comment on Arpaio's allegations.
The Arizona Democratic Party says in a statement that Arpaio's investigation is intended to draw attention away from problems within his own agency, such as hundreds of sex-crimes cases that the sheriff's office failed to adequately investigate over a three-year period.
So-called "birthers" maintain Obama is ineligible to be president because, they contend, he was born in Kenya.
Hawaii officials have repeatedly confirmed Obama's citizenship.
Obama released a copy of his long-form birth certificate in an attempt to quell citizenship questions.
Courts have rebuffed lawsuits over the issue.