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.

On Nov. 9, the day after Trump was elected president, Gene Hamilton, a member of Trump’s presidential transition team, emailed Kobach with what he called “Day One immigration policy action items.” Kobach responded by saying he had already begun work on proposed legislation targeting the NVRA.

“We will... be putting together information on legislation drafts for submission to Congress early in the administration,” Kobach told Hamilton in an email. “I have some already started regarding amendments to the NVRA to make clear that proof of citizenship requirements are permitted (based on my ongoing litigation with the ACLU over this).”

In a July 7 email to the ACLU also filed as part of court documents, Garrett Roe, Kobach’s attorney, noted the November email only referred to the possibility of a draft NVRA amendment and that the draft was never sent.

The email lends fuel to accusations by critics that Kobach and the Trump administration could be using the Presidential Advisory Commission on Election Integrity, of which Kobach is the vice chair and de facto leader, as a pretext for changing the NVRA with the aim of making it more difficult for Americans to vote.

Kobach did not immediately respond to a request for comment for this article.

ACLU legal filing In a Nov. 9 email to the Trump transition team, Kris Kobach explained he had already begun work on an amendment to the 1993 National Voter Registration Act.

In 2011, Kansas passed a law requiring voters to show proof of citizenship when they registered to vote. The ACLU sued Kobach over the law, saying it violated the NVRA, and federal courts stepped in last year to block it. A ruling by the 10th U.S. Circuit Court of Appeals in October forced Kansas to register more than 20,000 voters.

On June 28 of this year, the same day Kobach sent a letter to all 50 states requesting publicly available voter data, the Department of Justice sent a letter to the 44 states covered by the NVRA, asking them to detail their compliance with the voter-purging procedures outlined in the law. Voting advocates said the DOJ letter was deeply alarming, with some interpreting it as a signal that the department is gearing up to sue states to kick people off the voting rolls.

The November email is the first piece of evidence of Kobach’s intent to amend the NVRA ― something that critics have suspected since Kobach was photographed last November holding documents that listed potential alterations to the law. Kobach initially refused to turn over those documents to the ACLU in the lawsuit over Kansas’ proof-of-citizenship law, only doing so once a judge forced him to. The judge also sanctioned him with a $1,000 fine for making “patently misleading representations” about the documents to the court.

Kobach marked the documents as confidential, but the ACLU filed a motion in court Friday to have them unsealed.

Kobach, who was an immigration policy adviser during Trump’s White House transition, has been the focus of national attention as the work of the voter fraud panel begins. Election officials in nearly every state have either refused or said they cannot fully comply with Kobach’s request for publicly available voter data.

The panel’s first meeting is scheduled for Wednesday, July 19.