When top members of Donald Trump’s team add the word “period” to their most outlandish claims, it’s a safe bet they know they’re lying. The day after the president’s inauguration, for example, then-White House Press Secretary Sean Spicer angrily told reporters, “This was the largest audience to ever witness an inauguration – period.”

This came to mind yesterday afternoon.

Homeland Security Secretary Kirstjen Nielsen took to Twitter to vehemently deny claims that her department’s border policy dictates separation of children from their families. “We do not have a policy of separating families at the border. Period,” Nielsen tweeted late Sunday.

Let’s recap where the Trump administration stands on this:

Donald Trump: Everyone should blame Democrats for the policy.

Stephen Miller: Actually, we love the policy.

Jeff Sessions: Not only do we support the policy, but the Bible justifies the policy.

Melania Trump: It’d be nice if “both sides” got together to fix my husband’s policy.

Kirstjen Nielsen: There’s a policy?

NBC News’ Benjy Sarlin, trying to find a way to give the DHS chief the benefit of the doubt, added, “Technically, you could say ‘Well, it’s not a family separation policy, it’s a policy of prosecution that then results in some families being separated.’ But that doesn’t really do it justice either, in part because some in the White House explicitly say the separation itself is a deterrent.”

I can appreciate the fact that Nielsen, a curious choice for this cabinet post given her previous failures, is in a difficult position. Trump recently spent a half-hour berating her in front of the entire cabinet, apparently blaming her for the lack of progress on his immigration agenda, and reportedly prompting Nielsen to consider resigning.

But that’s not much of an excuse for the Homeland Security secretary doing lasting harm to what’s left of her credibility.

Earlier this year, Nielsen gave Senate testimony in which she said she didn’t know that Norway’s population is largely white. A few months later, she said she was completely unaware of Russia favoring Trump in the 2016 presidential election. Now, the cabinet secretary is pretending the administration’s family-separation policy doesn’t exist.

Circling back to a point we discussed several months ago, it’s easy to forget just how massive the Department of Homeland Security is. The nation’s newest cabinet agency, created in the wake of the 9/11 attacks, has nearly a quarter of a million employees, tackling a wide variety of tasks: DHS includes everything from FEMA to Customs and Border Protection to the Secret Service.

It’s therefore important for Americans to have confidence, not only in the department, but in its leadership. And while I hope there are no major national crises during Nielsen’s tenure, I wonder what would happen if she has to brief the public about an emergency, and many Americans are left to wonder whether the secretary can be counted on to tell the truth.