IOWA CITY, Iowa (AP) — Iowa Secretary of State Matt Schultz's push to uncover voter fraud has yet to lead to any criminal charges, but he says investigators still are looking into suspected instances of double-voting and non-citizens casting ballots.

Schultz, a Republican serving his first term as the state's top elections official, has made it his top priority to persuade lawmakers to pass a law requiring voters to show identification at the polls. He says doing so would prevent what he calls cheating. But critics, including the Democratic Party of Iowa, have said the state runs clean elections and the identification requirement would disenfranchise voters. Many local elections officials say fraud is not a problem.

Against that backdrop, Schultz has promised in recent months that investigations by his office would prove his assertions. He told the Iowa Republican, an online news site, on Feb. 29: "We will be showing that there are cases of voter fraud in Iowa." At a Republican Party dinner in May, he told activists he was "turning over every stone."

"I can tell you that, very soon, a clear message will be sent that you can't cheat in Iowa. We are looking at our records," he said. "I can tell you that we are coming on to something."

But records released by Schultz's office last week, in response to a request from The Associated Press, show investigators could not find fraud in three cases involving voters whose qualifications were questioned after November's elections. In two cases, the voters were found to have been legally registered. In the third, a foreigner living in the U.S. apparently voted improperly but an investigator said he did not have criminal intent.

"I would like to emphasize that the individuals reported to our office were not found to have participated in any intentional wrongdoing that could be classified to the level of 'fraudulent'," Angela Davis, the staff attorney in Schultz's office, wrote to AP, which asked for records related to all voter fraud investigations.

Davis said the details of those three cases could be released because they have been closed. But she withheld records involving several other cases, saying they've been sent to the Iowa Division of Criminal Investigation and local prosecutors for investigation.

Schultz said in an interview that his office has uncovered potential cases of persons who are not U.S. citizens voting and voters casting ballots in multiple states in the same election. He declined to elaborate, citing the ongoing investigations. But he said he was confident charges would be announced sometime soon.

"These things take time and you don't want to accuse people of things they haven't done," he said. "It's important to not just throw things out there. It's important to do your due diligence and go through the investigation process."

Schultz has opened a "voter fraud" telephone hotline for anyone to report irregularities, and his agency's website prominently features a place to do the same. He said Iowa elections are getting "closer and closer" and even a few illegal votes could change outcomes. He pointed to the Iowa caucuses in which Rick Santorum beat Mitt Romney by a few dozen votes and razor-thin state Senate elections.

Of the three closed cases, one of them involved an Iowa Falls man who voted even though he is not a U.S. citizen. The 38-year-old man was allowed to register to vote after providing his alien registration number, which is on the green card allowing him to live and work in the U.S.

"I did not get the impression that there was any intent to vote in a fraudulent manner," Iowa Falls Police Capt. Dave McDaniel wrote in February.

Another involved a DeSoto woman who registered at an address where she lives but does not receive mail. A postcard sent by local elections officials to confirm her address was returned as undeliverable. The woman was later visited by a sheriff's deputy who confirmed "she lived at the address listed on her voter registration form."