In many ways, Homeland Security Secretary Kirstjen Nielsen has been the face of family separation. When uproar over the program reached a peak in June, Nielsen took to the White House Briefing Room to defend the administration’s “zero tolerance” policy of prosecuting undocumented parents for illegal entry and taking away their children. One day later she was heckled out of a Mexican restaurant in Washington, D.C. by angry protesters.

At the same time, though, Nielsen has remained insistent that the U.S. government had no actual policy of purposefully separating families. Her position has always been that the separations were a side effect of the administration applying previously unenforced criminal immigration law.

When asked in that press briefing if family separation was intended as a deterrent, Nielsen responded unequivocally: “I find that offensive. No, because why would I ever create a policy that ever does that.”

Despite the facts—that thousands of children were forcibly taken from their parents at the border and multiple other administration officials praised the policy’s deterrent value—Nielsen has reiterated this position constantly.

“We do not have a policy of separating families at the border. Period,” Nielsen tweeted the day before that White House briefing.

In sworn Congressional testimony over the course of the past 10 months, she has been equally adamant that no such policy existed.

“When we separate, we separate because the law tells us to, and that is in the interest of the child,” Nielsen told the House Homeland Security Appropriations Subcommittee in April.

“We do not have a policy to separate children from their parents,” Nielsen said one month later in response to questioning from Sen. Kamala Harris before the Senate Homeland Security and Governmental Affairs Committee.

Finally, in an almost farcical appearance before the House Judiciary Committee just last month, Nielsen repeatedly rejected the premise of Democratic questions about the “family separation policy,” stating over and over again that there was no such policy.

Last week, NBC published further evidence that there was such an official policy and that Nielsen has been lying to Congress. The new proof came in the form of a late 2017 draft memo outlining “policy options” for responding to undocumented immigration. Those options included a plan to “Separate Family Units” and another plan to “Increase Prosecution of Family Unit Parents” and place “minors present with them … in HHS custody as [unaccompanied alien children],” e.g., to separate children from their parents.

It’s unclear whether the former plan was ever officially enacted. The latter proposal, though, had been piloted “on a limited basis in the El Paso Sector.” The next proposed step was for Nielsen to officially sign off on that plan in a further memo, which she appears to have done on April 23, 2018.

This newly revealed memo also makes explicitly clear that the purpose of zero tolerance and its subsequent family separations was for the practice to be “reported by the media and … have a substantial deterrent effect.” In her sworn responses to Harris in May, the Homeland Security Secretary specifically said “I have not been directed to [separate parents from children] for purposes of deterrence, no.”

Depending on what she knew about this memo and when she knew it, the “substantial deterrent effect” line is as close to a smoking gun as possible to demonstrate that Nielsen has been lying to Congress.

Indeed, Sen. Jeff Merkley, who provided the document to NBC after receiving it from someone he described as a whistleblower, has called on the FBI “to investigate [whether Nielsen] committed perjury when she testified before Congress.”

While the Trump Justice Department is unlikely to prosecute Nielsen, Merkley’s allegation is a serious criminal accusation. Given the growing evidence, why would Nielsen have subjected herself to the risk of such a charge by repeatedly misleading Congress about family separation and its intent?

As I observed in June, Nielsen’s adamancy appeared rooted in the then-pending litigation against the policy. Defending the practice before a federal judge in early May, a Department of Justice attorney insisted that there was “not such a policy” of “separation of families as a deterrence mechanism” and acknowledged that such a practice would be constitutionally dicey. (The judge very much agreed.)

Nielsen may have had to maintain the fiction that there was no policy while the government was trying to win that court case, and then stand by it even after the government lost.

Nielsen might also argue that she couched her statements to Congress in enough caveats for them to be technically true. In December, for example, she told members of the House Judiciary Committee “there is no comprehensive policy to separate families.” “Comprehensive” and “is” might be doing a whole lot of legal work there.

Either way, we should soon find out Nielsen’s defense: Three newly empowered Democratic chairmen of relevant committees responded to the memo’s release by demanding Nielsen testify about family separation and “her inaccurate statements to Congress and the American people.”

Whatever her explanation is, it better be good.