The Department of Homeland Security released Tuesday a study that sheds new light on the connection between the U.S. immigration system and so-called homegrown terrorism.

Of the 549 people convicted of international terrorism-related charges between Sept. 11, 2001 and and the end of last year, 402 — 73 percent — were foreign-born, according to the DHS report. Of those, 148 had become naturalized U.S. citizens before committing terrorism offenses.

DHS Secretary Kirstjen Nielsen says the findings suggest the U.S. needs to improve its immigration vetting process, to include screening certain immigrants after they arrive.

“I think what we take directly away from the report is we need to continue to enhance our screening and vetting,” she told CBS’ John Dickerson on Tuesday. “But it also tells us we need to continually vet those who are here. We have examples unfortunately over the last decades of terrorist attacks from legal permanent residents and others who were naturalized.”

The DHS study follows a spate of recent terror-related offenses allegedly committed by foreign-born people who had immigrated to the U.S. through legal channels. The suspects two incidents in New York City — a deadly truck rampage in October and an attempted suicide bombing in December — had arrived through the diversity visa lottery and on a family preference visa, respectively.

Last month, a naturalized U.S. citizen from Pakistan named Zoobia Shahnaz was indicted for allegedly laundering more than $85,000 through Bitcoin and other cryptocurrencies to fund Islamic State fighters overseas. Shahnaz came to the U.S. came to the U.S. on an F43 family-preference visa, which is granted to the children of siblings of U.S. citizens, according to DHS. (RELATED: DHS: Suspects In Terror-Related Crimes Came To US Through Chain Migration)

Trump administration officials have pointed to the incidents as evidence of public safety and national security vulnerabilities in the U.S. immigration system. Shortly after taking office, Nielsen backed Attorney General Jeff Sessions — the administration’s leading immigration hawk — in calling for tighter limits on extended-family migration and ending the green card lottery.

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