Secretary of Homeland Security Kirstjen Nielsen told reporters on Monday the exact number, which NBC News is first to report, was classified but that she was working on making it public.
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Overall, 41 people on the Terrorist Screening Database were encountered at the southern border from Oct. 1, 2017 through March 31, 2018, but 35 of them were U.S. citizens or lawful permanent residents. Six were classified as non-U.S. persons.
On the northern border, CBP stopped 91 people from the database, including 41 who were not American citizens or residents.
The White House has used the 4,000 figure to make its case for building a wall on the southwest border and for closing the government until Congress funds it. They have also threatened to call a national emergency in order to get over $5 billion in funding for the wall.