More than 300 people admitted to the US as refugees are being investigated by the FBI for potential terror-related activities, says the top law official.
Attorney General Jeff Sessions pointed to this FBI probe to justify the new executive order banning travel from six mainly Muslim countries.
The order, which puts a 90-day travel ban on Iran, Libya, Syria, Somalia, Sudan and Yemen, begins on 16 March.
But Mr Sessions did not say how many of the 300 came from the banned countries.
Nor did he say what the alleged offenses are or how many may face charges.
A spokeswoman for the American Civil Liberties Union told the BBC it was an assertion that "leaves many of us scratching our heads".
"The Trump administration has offered up no proof behind this assertion, making it impossible to evaluate this claim without more information."
Trump signs new travel-ban directive