DENVER – Colorado Secretary of State Wayne Williams said Friday that his office has found no evidence to back up accusations leveled this week by a Republican candidate for governor that a rival’s petition signatures were gathered illegally and should be thrown out.

“We have not seen any evidence” proving alleged wrongdoing by petition circulators working for GOP front-runner Walker Stapleton, Williams told Colorado Politics.

But Republican gubernatorial candidate Doug Robinson, a retired investment banker and nephew of Mitt Romney, said he’ll continue pressing election officials to determine whether petition circulators employed by Stapleton, currently state treasurer, skirted requirements.

“We worked extremely hard to make sure that our signatures and our circulators were valid, so it’s frustrating to see another campaign be so brazen in their violation of the rules,” Robinson told Colorado Politics. “We need integrity in our election process. We can’t simply brush this aside.”

A spokesman for Stapleton’s campaign dismissed Robinson’s allegations Friday, and the head of the Colorado Springs-based firm the Stapleton team hired to collect signatures, Kennedy Enterprises, said everything was handled properly.

Meanwhile, the executive director of the liberal ProgressNow Colorado advocacy organization compared the allegations to a 2016 scandal involving a Republican U.S. Senate candidate’s forged signatures and called on Attorney General Cynthia Coffman – another Republican candidate for governor – to investigate.

The controversy arises midway through the signature verification process that will determine whether candidates qualify for the June primary ballot.

Williams’ office is reviewing petitions submitted by Stapleton, Robinson and businessman Victor Mitchell, a third Republican running for governor. To make the ballot, they must gather 1,500 signatures from registered Republicans in each of Colorado’s seven congressional districts – a total of 10,500. (Another half dozen Republicans are trying to win a spot on the ballot at the Republican state assembly on April 14.)

Robinson’s charges stems from a Feb. 23 complaint filed with Williams’ office by the firm he hired to gather signatures describing “alarming interactions” with circulators for Stapleton’s campaign.

In one such alleged interaction, a circulator purportedly named Daniel Alejandro Velasquez, a Miami resident, said he was collecting signatures without being registered to vote in Colorado and had turned over completed petition pages without certifying them, contrary to legal requirements.

Dustin Olson, a co-founder of the firm conducting Robinson’s petition drive, told Colorado Politics he later recorded a nearly half-hour phone call with the circulator after election officials in Williams’ office asked him to gather evidence to support his firm’s suspicions.

“We were given a choice of turning a blind eye to blatantly illegal activity or doing the right thing,” Olson said.

“Most of my people can’t even register to vote because they’re all felons,” the man identified as Velasquez can be heard saying as he gathers signatures. The recording, first reported by television station KMGH-Denver7, was reviewed by officials at Williams’ office.

But that’s where Williams and the firm hired by Stapleton say the allegations hit a brick wall.

“I had never heard of Daniel Alejandro Velasquez until this story came out from the Robinson campaign,” Dan Kennedy, head of Kennedy Enterprises, said in an email. “All of the petition circulators for the Walker Stapleton signature campaign met all the requirements set forth by the Colorado Secretary of State, and all the signatures were collected legally.”

Williams told Colorado Politics Friday that his staff looked into the allegations but couldn’t find any evidence of wrongdoing.

“Dustin did a telephonic fake job interview with a petition circulator who gives his name, says he’s a three-time felon and makes statements he’s not following Colorado law. There are no petitions turned in from the Stapleton campaign from this individual. Zero. If there are, then I’d have something very specific to follow up on,” Williams said, adding that his office has attempted to contact the man allegedly named Velasquez without success.

“We have not seen any evidence that indicates any of the actual circulators, as opposed to the disreputable individual who was trying to get a job – since he says he is not following the law entirely, and he admits he is a three-time felon, I don’t view him as an entirely credible individual,” Williams added. “We’ve investigated to the extent we can. If someone has [additional] evidence someone wasn’t doing things properly, we’ll look into it.”

A spokeswoman for Robinson’s campaign said the candidate hasn’t ruled out asking a prosecutor to investigate the allegations but believes Williams should look into it further.

“The secretary of state’s job is to uphold the integrity of the election process,” Brett Maney said in an interview. “From our perspective, that requires that his office do his due diligence in looking into this matter.”