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."