In his interview with CBS News that aired Sunday, President Trump reiterated one of his frequent arguments for building a wall on the border with Mexico.

“This really is an invasion of our country by human traffickers. These are people that are horrible people bringing in women mostly, but bringing in women and children into our country. Human trafficking,” Trump said. “And we’re going to have a strong border. And the only way you have a strong border is you need a physical barrier. You need a wall.”

He’s made similar claims in the past, often telling evocative tales of vans sneaking across the border, their cargo areas filled with women bound and gagged with electrical tape. The tape is blue, he has added at times, a bit of detail seemingly meant to make the anecdote more believable.

It isn’t. The Washington Post looked into Trump’s tales, trying to find evidence that such incidents occurred at all, much less with regularity. We came up empty. Not only did we come up empty on that hunt, but also Vox later reported that the Border Patrol scrambled to find such stories after our story was released. So far, none has been presented. Customs and Border Protection will frequently issue news releases documenting the capture of people being smuggled into the country illegally, but those are incidents of attempts to illegally migrate, not of people being carried across the border for sex trafficking.

In his Sunday interview, this claim about trafficking was essentially the entirety of Trump’s rhetoric in support of the wall. CBS’s Margaret Brennan, though, didn’t ask the natural follow-up question, one that the administration doesn’t seem to have answered at all:

How much of this trafficking takes place?

When the Department of Homeland Security put together a slide deck for Congress of information about the need for a wall on the border, it didn’t include any metrics about human trafficking. Trump himself has never proposed a figure; an email to a Homeland Security spokesperson on Monday was not answered.

Last June, Homeland Security Secretary Kirstjen Nielsen gave a speech in which she offered one metric for human trafficking.

“Again, let’s just pause to think about this statistic: 314 percent increase in adults showing up with kids that are not a family unit,” she said. “Those are traffickers, those are smugglers, that is MS-13, those are criminals, those are abusers.”

That 314 percent increase, it turned out, stemmed from a specific bit of data. When migrants arrive at the border, they fall into one of three categories: Adult individuals, family units and unaccompanied minors. The former and latter are self-explanatory. Family units are groups purportedly including a parent and a child. Sometimes, though, the adult accompanying a minor isn’t a parent, and sometimes, they are pretending to be to facilitate entry into the United States.

Nielsen’s increase reflected a jump from 46 fraud cases in fiscal 2017 (“individuals using minors to pose as fake family units”) to 191 in the first five months of 2018. This was a big increase in the abstract — but it made up 0.61 percent of the family units who came to the border in that five-month period. What’s more, there is no indication that most or even many of these cases involved “traffickers,” “criminals” or “abusers.” Some may have been, for example, friends or neighbors who were asked by families to bring children with them on the journey north to protect them from violence.

Later, new numbers were reported by Paul Bedard of the Washington Examiner. In the last six months of the year, he said, “507 aliens were encountered as a family unit” and were “separated as they were not a legitimate family unit.” Many of those were because they didn’t meet the government “family unit” definition, including whether the children were older than 18. The number who weren’t related totaled 170 — apparently a drop from the first five months of the year.

Thanks to a surge in family units arriving at the border at the end of the fiscal year, those 170 made up 0.25 percent of all of those units who came to the United States in that period.

The question of how many people are trafficked and forced into the sex trade is not a new one. In 2007, The Post reported that despite claims of tens of thousands of people being trafficked into the United States each year, the Bush administration had determined that about 1,400 had been since 2000.

A report released in June of last year calculated that there had been about 8,300 people from 2011 to 2015 referred for criminal prosecution for a broad range of human trafficking offenses. About 4,700 were prosecuted during that period.

The map above shows where criminal referrals came from across the United States — a distribution that doesn’t seem to correlate to proximity to the border with Mexico. Information about the victims wasn’t included in the report, but in 2015, about 94 percent of the 964 criminal defendants in trafficking cases were U.S. citizens.

Even if we had hard numbers delineating how many people were in the United States as a function of sex or human trafficking, to evaluate Trump’s rhetoric on the wall, we would need to know how many came in from Mexico and, further, how many came in at places where a wall would have prevented their entry. The Customs and Border Protection website is littered with stories about people being caught hiding in vehicles that were entering the country — often at existing border checkpoints. Trump’s argument that smugglers drive through the desert until the wall ends and then head north seems to be undercut by the fact that so many smugglers simply try to walk through the existing doors.

Trump’s rhetoric on the wall has long been riddled with holes. In this case, though, he’s explicitly embracing an undercurrent to his claims about drugs and terrorism: that this unseen, unholy thing is happening, and we must stop it. It’s a nifty bit of rhetoric, evoking strong emotions while also finding strength in the lack of evidence for its existence: Maybe the smugglers are so good that we can’t even catch them doing these horrible things in unwalled areas!

Or perhaps a wall wouldn’t do much, if anything, to curtail this particular horrible crime.