The source is a report from the
Crime Prevention Research Center, used a previously untapped set of data from Arizona that detailed criminal convictions and found that illegal immigrants between 15 and 35 are less than 3 percent of the state’s population, but nearly 8 percent of its prison population. And the crimes they were convicted of were, on the whole, more serious, said John R. Lott Jr., the report’s author and president of the research center.
Link to the report: Undocumented Immigrants, U.S. Citizens, and Convicted Criminals in Arizona by John R. Lott :: SSRN
I assume you read a blog?
Your link is a biased opinion piece which exaggerates the "small error" which was already noted in the report. "While it is possible that some of the convicts who are listed as US-born and as either undocumented or documented immigrants renounced their citizenship or never had their citizenship claimed for them, it is also possible that there are data errors. Even if all these cases are in error, the number of cases is still small and they don’t appreciably alter the results."
Arizona Department of Corrections knows each prisoner who entered the prison system, their criminal history, and whether he is a documented or undocumented immigrant. Undocumented immigrants have the highest rates, whereas documented immigrants actually have lower rates than do U.S. citizens.
Link to the report: Undocumented Immigrants, U.S. Citizens, and Convicted Criminals in Arizona by John R. Lott :: SSRN