(WASHINGTON) — Democratic Sen. Jeff Merkley on Friday asked the FBI to open a perjury investigation into Homeland Security Kirstjen Nielsen following the leak of an internal draft 2017 document that detailed the administration's legal options for separating families as a way to curb illegal immigration at the border.

The memo challenges the assertion by Nielsen last June on Twitter and in media briefings saying "we do not have a policy of separating families at the border. Period."

The Dec. 16, 2017, document, provided by Merkley's office, which said it received it from a whistleblower, appears to be an internal memo on legal options to address families who were arriving at the border. Among the options listed: the "prosecution of family units" and "separate family units."

We do not have a policy of separating families at the border. Period.

— Sec. Kirstjen Nielsen (@SecNielsen) June 17, 2018

"Announce that DHS is considering separating family units, placing the adults in adult detention, and placing the minors under the age 18 in the custody of (Health and Human Services) as unaccompanied alien children," states one option in the document.

Merkley notes that Nielsen made similar comments in congressional testimony, at one point telling the House Judiciary Committee: "I'm not a liar. We've never had a policy of family separation."

"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, from Oregon, wrote in a letter to FBI Director Christopher Wray.

On Thursday, a spokeswoman for DHS said the document was intended to examine the administration's options. The spokeswoman did not immediately respond to Merkley's request for a perjury inquiry.

“The Trump administration has made clear that all legal options are on the table to enforce the rule of law, rein in mass unchecked illegal immigration, and defend our borders," said spokeswoman Katie Waldman in an emailed statement.

Waldman said the administration saw an uptick of border arrests, which it blamed on a lack of a border wall and a court settlement that prohibits detention of children for longer than 20 days. Officials have said previously they believe that court settlement, known as the Flores Agreement, encouraged people to travel with minors.

"In part we were predicting — and trying to prevent — the exact humanitarian and security crisis we are confronted by now," Waldman said. "It would be malpractice to not seriously examine every single avenue to gain operational control of the border and ensure that those who are entering our country have a legal right to be here."

Shortly after the December 2017 memo was drafted, in April 2018, the Trump administration announced a "zero-tolerance" policy at the border that resulted in the separation of some 2,700 kids from their families in a matter of weeks.

In court filings, the administration has estimated a total of 2,737 children were separated from their families under "zero tolerance." But a report released Thursday by internal government investigators found that there were likely "thousands" more kids separated from families in 2017, before that policy took effect. The Health and Human Services inspector general said that the number of kids separated from their families under the current administration is "unknown."

While enforcement deterrents like the ones described in the memo have been effective in stopping adult men from entering further into the U.S. for economic reasons, experts at the nonpartisan Migration Policy Institute say those methods don't address the current humanitarian crisis.

"There is no evidence that such deterrence tactics work against the humanitarian flows today," MPI analyst Sarah Pierce told ABC News. "So not only is it immoral, but it is misguided and foolish."

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