National correspondent

All right. Settle in for a bit and maybe swipe over to the calculator app on your phone. We’re about to go on quite a numeric roller coaster ride.

President Trump shifted the gears of his immigration rhetoric a bit Friday, hoping to reclaim the momentum lost after having to publicly backtrack on his administration’s family-separation policy. He returned to a tactic he first embraced less than a month after declaring his candidacy: standing with the parents of people killed by immigrants in the country illegally to suggest that immigrants pose a significant threat to Americans.

The data suggests that immigrants, including immigrants in the country illegally, commit crimes at lower rates than native-born Americans. But Trump has consistently rejected that idea, instead choosing to hype vague assertions that immigrants are more dangerous.

As at Friday’s event, when he waved away the demonstrable fact that immigrants commit fewer crimes by saying: “You’ve heard that, fellas, right? You’ve heard that. I hear it so much, and I say, ‘Is that possible?’ ”

Well, he said: “The answer is it’s not true.”

It is.

Later on, though, Trump echoed a statistic of his own.

“Sixty-three thousand Americans since 9/11 have been killed by illegal aliens,” an attendee said. “This isn’t a problem that’s going away; it’s getting bigger”

“Sixty-three thousand, and that number they say is very low because things aren’t reported,” Trump replied. “Sixty-three thousand, and you don’t hear about that.”

No, you don’t. Because it’s demonstrably not true, and it wends its way back to a blog post from 2005 written by none other than Rep. Steve King (R-Iowa).

Trump first came across the figure in March. He was hosting a roundtable on sanctuary cities at the White House that included, among other guests, Mary Ann Mendoza.

Mendoza is the mother of Sgt. Brandon Mendoza, killed in 2014 by a drunk driver, an immigrant in the country illegally who had had previous charges against him dismissed. At that roundtable, Mary Ann Mendoza presented Trump with the 63,000 number.

“Over 63,000 Americans have been killed since 9/11 by illegal aliens,” she said. “It’s a crime spree that is being left unchecked, and these sanctuary city officials — state and city officials — they’re putting American lives into harm every single day.”

“I never heard the number 63,000 people killed by illegal immigrants,” Trump said in response. “Is that a known figure? Is that an acknowledged —”

“It’s an average of 12 Americans a day, which is how many are killed in the United States,” Mendoza replied.

“Boy. That is some number,” Trump said. “I’ve never heard that number before. That’s an incredible number.”

It is incredible, quite literally.

Consider that there were an estimated 17,250 murders in the United States in 2016, the most recent year for which full FBI data is available. Mendoza’s per-day number would indicate that there were 4,380 murders committed by immigrants in the country illegally — a full quarter of those killed that year. (It would also mean that her number was a bit out-of-date: On March 20, 2018, 6,034 days had passed since the 9/11 attacks, meaning 72,408 alleged murders.)

We traced that number back to its source. On May 5, 2006, King posted an article on his official website in response to national pro-immigration protests held that month. The post is a catalogue of anti-immigrant scaremongering. Here’s the key paragraph, with emphasis added:

“What would that May 1st look like without illegal immigration? There would be no one to smuggle across our southern border the heroin, marijuana, cocaine, and methamphetamines that plague the United States, reducing the U.S. supply of meth that day by 80%. The lives of 12 U.S. citizens would be saved who otherwise die a violent death at the hands of murderous illegal aliens each day. Another 13 Americans would survive who are otherwise killed each day by uninsured drunk driving illegals. Our hospital emergency rooms would not be flooded with everything from gunshot wounds, to anchor babies, to imported diseases to hangnails, giving American citizens the day off from standing in line behind illegals. Eight American children would not suffer the horror as a victim of a sex crime.”

It was an expansion of an argument he had made from the House floor two days after the May Day protests. (In that speech, the abused American children were “at least eight little girls.”)

There, he explained the math behind his numbers.

“The crimes that are committed by those who enter this country illegally are in significantly greater numbers than the crimes that are committed by American citizens,” King said, “to the extent that 28 percent of the inmates in our prisons in the United States are criminal aliens, 28 percent.”

That figure he got from a Government Accountability Office report issued in April 2005. It read:

“At the federal level, the number of criminal aliens incarcerated increased from about 42,000 at the end of calendar year 2001 to about 49,000 at the end of calendar year 2004 — a 15 percent increase. The percentage of all federal prisoners who are criminal aliens has remained the same over the last 3 years — about 27 percent.”

So, King figured (admitting that the number would go up or down a percentage point or two):

” That means then that criminal aliens are committing 28 percent of the crimes in the United States. And so that means 28 percent of the murders, 28 percent of the rapes, 28 percent of the violence and the assaults and battery, first- and second-degree murder and also manslaughter attacks are committed by criminal aliens.”

In other words, since 28 percent of the prisoners were immigrants in the country illegally, they must also therefore have committed 28 percent of each and every crime! Quod erat demonstratum. Recognizing that this sounded like a lot even at the time, King rationalized it by asserting that for every undocumented immigrant in the country we know about, two or three sneak in without our knowing it. Here’s how we know that’s not true.

There is an almost impossibly large number of problems with King’s argument.

The first is that, of course, crime is not evenly distributed among populations. If 10 percent of prisoners are named John, that doesn’t mean that people named John commit 10 percent of murders. Many of those undocumented immigrants in federal custody were probably there for a very specific type of crime: felony immigration violations. Very, very few native-born Americans were there for that. In fact, in 2014, half of criminal arrests were for immigration violations. Since the GAO data are only for federal prisoners, that’s significant. (It’s also worth noting, as a reader did, that federal prisoners make up only a small percentage of the entire prison population.)

What’s more, the GAO data dealt with “criminal aliens” — defined in the report as “noncitizens convicted of crimes while in this country legally or illegally,” emphasis again added. In other words, King’s 28 percent number included people who weren’t even immigrants who had entered the country illegally.

Put simply, King seems to have basically made the number up more than a decade ago and, since there isn’t good data on the number of crimes committed by people who immigrated illegally, it was embraced by those looking to put a number to it. There’s simply no credible reason to believe that there have been 63,000 killings committed by undocumented immigrants since 9/11, and plenty of reason to think that the homicide rate among members of that group is lower than for native-born Americans.

What’s really remarkable, though, is that this is a bit of data that was just presented as a matter of fact by the president of the United States. This was a number so impossibly useful for Trump that when he first heard it, even he was a bit wary about its provenance. And yet, three months later, he’s presenting it as accurate without a hint of an asterisk.

In 2011, King presented a resolution that offered a different calculus.

“According to the Government Accountability Office,” it read, “there have been 25,064 homicide arrests of criminal aliens since 1955.”

That’s about 1.2 per day.

This article was corrected to properly attribute the initial mention of 63,000 figure on Friday.