WASHINGTON (March 18, 2013)—U.S. Supreme Court justices seem conflicted over whether states can force people to prove their U.S. citizenship before registering to vote.

Arizona is challenging a lower court's refusal to let it demand that prospective voters provide proof of U.S. citizenship in order to use a federal registration form that doesn't require such proof.

A federal appeals court threw out that part of Arizona's Proposition 200, which was passed to stop illegal immigrants and other non-citizens from registering.

Several justices questioned whether the state can require information that the federal government did not require for federal voter registration while others questioned whether the government's only requirement that someone swear that they're a citizen was enough.