President Donald Trump greets people holding posters of victims allegedly killed by illegal immigrants, before speaking on immigration in the South Court Auditorium, next to the White House on June 22, 2018 in Washington, DC. Mandal Ngan | AFP | Getty Images

In his efforts to draw support for tougher immigration policies, President Donald Trump has repeated a shocking claim from activists: that 63,000 Americans have been killed since the 9/11 terrorist attacks by people who came to the U.S. illegally. But that statistic, which Trump has claimed is a conservative estimate, appears to be based on a flawed reading of government data from a 2011 study by the Government Accountability Office. "The actual use of the statistic appears to be either wrong or, at best, unjustifiable," said Regina Anne Bateson, an assistant professor of political science at the Massachusetts Institute of Technology who recently lost a bid for Congress in a California Democratic primary. The statistic, while drawing little from the actual report, "reflects a consistent effort to use big numbers to scare people," Bateson said. The White House did not immediately respond to CNBC's request for comment.

Trump and the 'angel families'

On Friday, Trump held an event for "angel families," those whose loved ones have been killed by immigrants who are in the U.S. illegally. Steve Ronnebeck, a guest at Friday's event who said his son was killed by an undocumented immigrant, was at least the second to claim to Trump that since Sept. 11, 2001, 63,000 Americans have been killed by people who entered the U.S. illegally. Trump, facing fierce pushback over his zero-tolerance immigration policy, seized on the figure: "63,000. And that number, they say, is very low because things aren’t reported," the president said from the stage.



Trump appears to have first heard 63,000 figure in March, at a roundtable discussion on so-called sanctuary cities from Mary Ann Mendoza, an activist who supports Trump's immigration policies. Mendoza, who identifies herself on Twitter as the "mother of police Sgt Brandon Mendoza killed by a repeat illegal alien criminal," has voiced the claim at least 12 times since March without citing a source. Mendoza's statistic, purporting to demonstrate a shocking level of danger from immigrants who are in the U.S. illegally, grabbed Trump's attention at the March roundtable. "I never heard the number 63,000 people killed by illegal immigrants. Is that a known figure?" Trump asked Mendoza. He quickly seemed convinced. "Boy. That is some number. I’ve never heard that number before. That’s an incredible number. Thank you, Mary Ann. Appreciate it very much," he said.

Math and that GAO report

Mendoza told CNBC that she extrapolated the figure from a 2011 study from the Government Accountability Office. But to get there, she relied on broad mathematical assumptions and fundamental misreadings of the variables, time frames and definitions in the data, according to CNBC's research. An estimate from the GAO's report on "Criminal Alien Statistics" analyzed the arrest records of 1,000 criminal aliens within a population of about 250,000 who were incarcerated in state and federal prisons as well as local jails between 2004 and 2008. The estimate found of the nearly 3 million arrest offenses cited from the study population, about 25,064 arrest offenses, or about 1 percent, fell under the category "homicide" for those four years. Mendoza divided that number by four to conclude that 6,266 homicides had been committed each year by undocumented immigrants. And since the report came out in 2011, Mendoza approximated that 10 years had passed between its publication and the 2001 terror attacks. Assuming that the number of homicide arrests would be equal every year led her to believe that 62,660 undocumented immigrants were arrested for homicide from 9/11 to 2011 — a number that is well above 63,000 now, she said.

A flawed statistic

But that methodology is inaccurate. The GAO report does not say, at any point, how many homicides were committed each year by undocumented immigrants. Rather, it estimated the number of "historical arrests" for the population of criminal aliens incarcerated in federal or state penitentiaries or local jails from 2003 to 2010, according to the agency's public affairs director, Charles Young, in a statement to CNBC. The arrests themselves "could have occurred between August 1955 and April 2010," Young said — roughly a 55-year gap. To be sure, the report notes that about 90 percent of the arrests in the study population occurred after 1990. For that reason, the GAO researchers "did not calculate [or] extract estimates of the number [of] homicide arrest offenses in a given year," Young said. Additionally, the term "criminal alien" is distinct from the term "illegal alien," the phrase used both times the statistic was cited to Trump. The GAO report defines criminal aliens as "noncitizens who are residing in the United States legally or illegally and are convicted of a crime." The numbers taken from the study refer only to criminal aliens, and the report does not distinguish between which noncitizens are in the country legally or illegally when the crimes are committed. It also does not say whether the homicide victims are all American citizens, or how many arrests map onto each homicide. In a follow-up conversation, Mendoza told CNBC that it is difficult to gather government data on homicides committed by immigrants who are in the U.S. illegally. "It's just so sad that we have to fight to get these figures and fight to prove what's happening to Americans," she said. Mendoza maintained, however, that she was "being very conservative" in her claim of 63,000 Americans killed, and suggested that the number is more likely twice as large at 125,000. She also questioned why the exact figure itself matters. "Why are we having this conversation? One person murdered by an illegal alien, unlawfully murdered in our country, is one too many."

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