Update, 2:30 p.m. Sept. 17: The Maricopa County Recorder's Office filed an emergency petition asking the Arizona Supreme Court to rule on whether the voters affected by this problem should be required to provide documented proof of citizenship before the November election in order to vote a full ballot.

The eligibility of nearly 100,000 registered voters in Arizona is up in the air because of an error in state systems uncovered just before the scheduled sending of mail ballots, Secretary of State Adrian Fontes announced on Tuesday.

The state incorrectly marked these voters when they registered to vote as already having provided documented proof of U.S. citizenship, when really, it's unclear whether they have, Fontes said.

The error stems from the way the Motor Vehicle Division provides driver's license information to the state's voter registration system. The voters affected by this particular error are people who first obtained their Arizona driver's license before October 1996 and then were issued a duplicate replacement 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. That means, if these voters have not provided the proof, they should be registered as what Arizona calls "federal only" voters and be allowed to vote only for president and Congress.

It's unclear why no one noticed the error sooner, Fontes said, but it's been occurring for roughly two decades and over four administrations.