HELENA, Mont. (AP) — Montana's chief elections officer alleged Thursday that voter fraud may be more common than local officials acknowledge and asserted that more than 360 illegal ballots were cast, but not counted, during the special congressional election in May.

Republican Secretary of State Corey Stapleton made the allegations before an interim legislative committee. He was immediately challenged by Sen. Sue Malek, a Missoula Democrat who leads the State Administration and Veterans' Affairs committee. She countered that those rejected ballots, a minuscule percentage of the 383,000 cast in the May 25 election won by U.S. Rep. Greg Gianforte, were rejected because of honest mistakes.

Stapleton acknowledged that no one in Montana has ever been convicted of voter fraud, but he suggested it routinely happens and urged county elections officials to take fraud allegations more seriously.

"I'm saying we have a system that can be improved," Stapleton said in an interview, adding that reports over fraud are too casually dismissed and "glossed over" by county elections clerks.

Stapleton has clashed with Missoula County elections officials over a complaint from one voter who alleged that her mail-in ballot may have been used by another voter. Elections officials said the incident might have been the result of a mix up in mail delivery. They set aside the earlier ballot and allowed the woman to cast another ballot.

However, Stapleton seized on the incident as an example of how potentially easy it could be to tamper with ballots.

State officials say more than 1,800 ballots were rejected statewide for various reasons in the congressional election. Of those, he said, more than 360 were rejected because of mismatched signatures. The disputed ballots represent .07 percent of the total vote.

"He's making allegations that are baseless and that he hasn't investigated," said Rebecca Connors, Missoula County's elections administrator.

Elections officials from across the state will gather next month in Anaconda for their annual convention and have urged the secretary of state to address his concerns by offering training on signature verification.