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The eligibility of nearly 100,000 registered voters in Arizona is uncertain because an error in the state’s systems was discovered shortly before the mail-in ballots were scheduled to be sent out, Secretary of State Adrian Fontes announced on Tuesday.

The state falsely labeled these voters during voter registration as if they had already provided proof of U.S. citizenship, when in reality it was not clear whether that was the case, Fontes said.

The error stems from the way the Department of Motor Vehicles transmits driver’s license information to the state’s voter registration system. The voters affected by this error are people who obtained their Arizona driver’s license before October 1996 and then obtained a replacement ID before registering to vote sometime after 2004, according to Fontes.

Arizona’s unique laws have required proof of U.S. citizenship to vote in state and local elections since 2005. This means that voters who cannot provide such proof are registered as “federal only voters” and are only allowed to vote for president and Congress.

It is unclear why no one noticed the mistake earlier, Fontes said, but it has been happening for about two decades and across four governments.

Gov. Katie Hobbs, who like Fontes is a Democrat, said she has directed the Department of Motor Vehicles to fix the problem and will “order an independent audit to ensure that the Department of Motor Vehicles’ systems are functioning as required to support voter registration.”

Fontes said a Maricopa County employee reported the issue after finding a person on the county’s voter rolls who had not provided proof of citizenship but had been included on the rolls as an at-large voter. It turned out the person had a green card and was not eligible to vote at all, Fontes said.

This person never cast his vote, Fontes said.

Maricopa County Recorder Stephen Richer confirmed in a statement that his team had raised awareness of the issue.

This discovery comes as Republicans are increasingly raising concerns about noncitizens voting in federal elections. There is no evidence that noncitizens are voting widely, and experts say such cases are exceedingly rare. Federal law requires voters to affirm their U.S. citizenship under oath when registering, but does not require documents proving it. Noncitizens who try to vote risk jail time, loss of immigration status and deportation.

Republicans in Congress are pushing to pass a law that would require proof of citizenship to vote in federal elections, similar to what Arizona does for state elections. Former President Donald Trump has said Republicans would have to be willing to shut down the federal government to get the law passed.

Arizona, a swing state where nationwide elections can be decided by a few hundred votes, is under special scrutiny in this year’s presidential election.

Time is running out: the ballot papers must go out soon

The state is unsure what to do next, knowing there may not be proper documentation on these voters and time is short. County officials must mail ballots to military members and voters overseas on Thursday, and mail-in ballots will be sent out in less than a month.

Maricopa County confirmed it will file a so-called “friendship lawsuit” in the Arizona Supreme Court on Tuesday, asking a judge to quickly order Fontes’ office to allow voters whose voter registration is questionable to vote for federal offices but not for state and local elections. Fontes said his office will argue that voters should still be given full voting rights.

“We don’t want that,” Fontes said about the possible restriction of this electorate. “We are in favor of the status quo.”

The roughly 97,000 voters who were mistakenly registered on the state’s voter rolls lean more Republican, Fontes said, and many are between the ages of 45 and 60.

The error in these voter registrations occurred because of the way the Department of Motor Vehicles categorizes driver’s licenses when it sends them to the voter registration system.

Since October 1996, Arizona residents have had to prove their U.S. citizenship to obtain a regular driver’s license. Noncitizens, such as green card holders, receive a different type of license. Previously, there was no such distinction.

Driver’s licenses in Arizona can remain valid for decades without needing to be renewed, an unusually long period of time. As long as the person lives in the state, their license will remain valid until the driver turns 65, and then it can be renewed every five years.

This means that many people who obtained their driving license before October 1996 still have it today.

Since the Proof of Citizenship for Voter Registration Act went into effect in 2004, when county officials receive a new voter registration with a driver’s license number, they check the license’s issue date. If the date is after October 1996, they know the person has already provided the state with documented proof of citizenship.

What officials didn’t know, however, was that the Department of Motor Vehicles changed the issue date on the driver’s license when someone requested a replacement ID or otherwise updated their ID, such as a change of address. This means that when someone who had received such a replacement license registered to vote, when election officials checked the license’s issue date, they would see a date after 1996 and believe the voter had provided documented proof of citizenship to obtain the license, even if they hadn’t.

These voters were then marked as voters who had cast the full vote, even though the state had not verified whether additional documentation was available.

Jen Fifield is a reporter for Votebeat based in Arizona. You can reach Jen at [email protected].