A Democratic senator has asked the FBI to investigate whether Homeland Security Secretary Kirstjen Nielsen lied to Congress about the Trump administration's family separations.

Sen. Jeff Merkley of Oregon released an internal 2017 document showing that top Department of Homeland Security officials discussed several "policy options," including one that would "separate family units."

Nielsen has denied that the Trump administration had a family-separation policy, saying that the only policy was to prosecute migrants for illegal entries and "uphold the laws that Congress has passed."

In December 2018, Nielsen testified to Congress that "we have never had a policy for family separation."

Sen. Jeff Merkley of Oregon announced Friday he had formally requested that the FBI investigate Homeland Security Secretary Kirstjen Nielsen, accusing her of lying to Congress about the Trump administration's family separations.

On Thursday, Merkley released a previously unseen document dated December 2017, showing that top Department of 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."

—Senator Jeff Merkley (@SenJeffMerkley) January 18, 2019

"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.

Read more: Trump officials keep contradicting each other trying to explain why they are separating screaming children from their parents at the border

The Department of Homeland Security did not immediately respond to INSIDER's request for comment.

A boy and father from Honduras are taken into custody by US Border Patrol agents. John Moore/Getty

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 said the administration's policy was 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.

"I'm not a liar. We have never had a policy for family separation," she told the House Judiciary Committee in December 2018.

Nielsen at a roundtable discussion on border security. Associated Press/Jacquelyn Martin

"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 provided false statements to Congress.