Sen. Jeff Merkley is requesting that the FBI open a perjury investigation into Homeland Security Secretary Kirstjen Nielsen, based on testimony she gave to Congress in December on family separations at the southern border.

Testifying before the House Judiciary Committee in December, Nielsen stated “I’m not a liar, we’ve never had a policy for family separation.”

Memos made public Thursday show that officials from the Departments of Justice and Homeland Security were exploring family separation polices as a deterrent for illegal immigrants a full year before that testimony.

The memo, which was titled “Policy Options to Respond to Border Surge of Illegal Immigration,” laid out options for speeding up deportation of migrant children by denying them their legal right to asylum hearings after separating them from their parents and families.

Merkley’s letter to the FBI is focused on that statement and the contrast with the memo, which is dated December 2017, a year before her testimony.

“In light of these conflicting facts, the FBI should immediately investigate whether Secretary Nielsen’s statements violate 18 U.S. Code § 1621, 18 U.S.C § 1001, or any other relevant federal statutes that prohibit perjury and false statements to Congress,” he wrote.

The practice of separating children from their families, part of a “zero tolerance” approach to immigration, sparked backlash last year as reports of children younger than five years old were placed in government custody and taken from their parents.

“The Federal Bureau of Investigation (FBI) has previously indicated that the Department of Justice requires a formal criminal referral from Congress to initiate an investigation concerning Congressional testimony. I write today to execute such a criminal referral,” wrote Merkley.