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 they were explained by a 95-year-old former state worker who signed the president's birth certificate. Zullo said a writer who published a book about Obama's birth certificate and was aiding investigators let them listen in on an interview he conducted of the former state worker.
The Obama campaign declined to comment on Arpaio's allegations.
The Arizona Democratic Party said in a statement that Arpaio's investigation is intended to draw attention away from problems within his own agency, such as hundreds of sex-crime 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, and state officials did again Tuesday.
"President Obama was born in Honolulu and his birth certificate is valid," Joshua A. Wisch, a special assistant to Hawaii's attorney general, said in a statement. "Regarding the latest allegations from a sheriff in Arizona, they are untrue, misinformed, and misconstrue Hawaii law."
Wisch also said that "not only are Hawaii's vital records some of the best managed, but they also have some of the strongest restrictions on access to prevent identity theft and fraud."
Obama released a copy of his long-form birth certificate in an attempt to quell citizenship questions.
Courts have rebuffed lawsuits over the issue.