Kansas Secretary of State Kris Kobach (R), the official leading President Donald Trump’s voter fraud probe, apparently sought to amend federal voting law to make it possible to require proof of citizenship during the voter registration process, according to an email made public Friday as part of an ongoing lawsuit with the American Civil Liberties Union.

The email offers the firmest evidence yet that Kobach was interested in amending the 1993 National Voter Registration Act, as has been speculated. The NVRA ― a law championed by voting advocates ― requires motor vehicle, public assistance and some other state agencies to provide voter registration opportunities, and outlines the procedure by which voters can be removed from the rolls.