U.S. Customs and Border Protection encountered only six immigrants at ports of entry on the U.S-Mexico border in the first half of fiscal year 2018 whose names were on a federal government list of known or suspected terrorists, according to CBP data provided to Congress in May 2018 and obtained by NBC News.

The low number contradicts statements by Trump administration officials, including White House press secretary Sarah Sanders, who said Friday that CBP stopped nearly 4,000 known or suspected terrorists from crossing the southern border in fiscal year 2018.

The Morning Rundown Get a head start on the morning's top stories. This site is protected by recaptcha

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. The data was the latest set on this topic provided to Congress. It is possible that the data was updated since that time, but not provided to Congress.

Overall, 41 people on the Terrorist Screening Database were encountered at the southern border from Oct. 1, 2017, to 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 listed in the database, including 41 who were not American citizens or residents.

Border patrol agents, separate from CBP officers, stopped five immigrants from the database between legal ports of entry over the same time period, but it was unclear from the data which ones were stopped at the northern border versus the southern border.

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.

The U.S. keeps databases of people it believes may have ties to terrorist networks based on their spending activities, travel patterns, family ties or other activities. It is not a list of people who could be criminally charged under terrorism statutes, and it is possible that someone could be stopped because they have the same name as a person on the list.