DENVER (AP) – Colorado Secretary of State Scott Gessler has mailed letters to nearly 4,000 registered voters asking them to verify they are naturalized citizens, or voluntarily withdraw their voter registrations.

Gessler’s office said Thursday the letters went to people who presented non-citizen documents like a green card when they applied for driver’s licenses and then registered to vote. It is illegal for non-U.S. citizens to vote.

Gessler says his office will work with county clerks to decide how to handle people who don’t respond to the letters.

The U.S. Department of Homeland Security had said it would expand access to an immigration database so that states can use it to remove non-citizens from voter rolls, but Gessler says his office is awaiting a final agreement from the federal government to use it.

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