A 1993 law easing voter registration for federal elections stipulates that voters must be U.S. citizens, but doesn't actually require voters to show proof of their eligibility.
The National Voter Registration Act requires that registrants swear under penalty of perjury that they are citizens. The lack of required proof conflicts with language in the constitutions of a handful of states, including Arizona. To comply with the law, upheld by the Supreme Court in 2013, Arizona maintains a separate roll of "federal only" voters. In 2018, there were 11,904 voters in that category, and 14% voted, according to the Arizona Mirror.
A federal court deemed similar citizenship requirements in Kansas, Alabama and Georgia unconstitutional in 2016.