On Thursday, Merkley released a previously unseen document dated December 2017, showing that top homeland security officials drafted a memo of "policy options" that explicitly discussed a policy to "separate family units."
The draft suggested that officials "announce that DHS is considering separating family units, placing the adults in adult detention, and placing the minors under the age of 18 in the custody of [the Department of Health and Human Services] as unaccompanied alien children."
"Compelling new evidence has emerged revealing that high-level Department of Homeland Security officials were secretly and actively developing a new policy and legal framework for separating families as far back as December 2017," Merkley said in a statement.
The Department of Homeland Security did not immediately respond to INSIDER's request for comment.
Though the Trump administration separated at least 2,737 migrant children from their parents at the height of its "zero tolerance" border policy in the spring, Nielsen has consistently denied that the administration had a policy to separate families.
Rather, she has argued, the administration's policy was only to prosecute all adult migrants who crossed the US-Mexico border illegally a process that de facto meant any children they brought with them had to be separated and sent to shelters across the country.
Nielsen has also repeatedly denied claims that she lied to the public about the family separations.
"A policy of family separation would mean that any family that I encountered in the interior, I would separate. It would mean that any family that I found at a port of entry, I would separate," she added. "It would mean that every single family that I found illegally crossing, we would separate. We did none of those. What we did do is uphold the laws that Congress has passed. And we prosecuted those who choose to come here illegally."
Merkley said in his statement that Nielsen's December 2018 testimony conflicted with the information found in the December 2017 memo.
"This policy, called 'Separate Family Units,' was specifically designed to gain media attention and generate a 'substantial deterrent effect,'" he said, adding that the FBI should "immediately investigate" whether Nielsen committed perjury and false statements to Congress.
