With as many as 1,000 active cases, Fox News has learned at least 48 ISIS suspects are considered so high-risk that the FBI is using its elite tracking squads, known as the mobile surveillance teams or MSTs, to track them domestically.

“There is a very significant number of people that are on suspicious watch lists, under surveillance,” said Sen. Dan Coats (R-Indiana).

Coats, who sits on the Select Committee on Intelligence, would not comment on specifics, but said the around-the-clock surveillance is a major commitment for the bureau. “The FBI together with law enforcement agencies across the country are engaged in this. It takes enormous amount of manpower to do this on a 24/7 basis. It takes enormous amount of money to do this,” Coats explained.

These elite FBI teams are reserved for espionage, mob violence and high-priority terrorism cases, like a joint terrorism task force case last June, where a 26-year-old suspect, Usaama Rahim, was killed outside a Massachusetts CVS. When a police officer and FBI agent tried to question him, the Boston police commissioner said, Rahim threatened them with a knife, and was shot dead, the New York Post reports.