The FBI is using elite tracking teams to track down at least 48 ISIS suspects in the U.S. who are considered high-risk.
"There is a very significant number of people that are on suspicious watch lists, under surveillance," said Republican Sen. Dan Coats, according to Fox News.
Coats, a member of the Select Committee on Intelligence, added, "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."
The elite teams are used for espionage, mob violence and high-priority terrorism cases like what happened to a case last June when 26-year-old suspect Usaama Rahim was killed outside a Massachusetts pharmacy.
The high level of surveillance, Fox News said, reflects a severe risk associated with suspects that may attempt copycat attacks like the ones staged in Paris.
"It is a big resource drain. Yes it is. Almost overwhelming," said Coats. "The threat is particularly high during this holiday period."
The report said the Islamic radicalisation process can be swift. Hasna Ait Boulahcen, the female suspect who was killed in the Sait Denis siege, withdrew from her partying ways and became an Islamic fanatic just a month after meeting her cousin, Abdelhamid Abaaoud, who introduced her to radical Islam, authorities learned. She later joined Abaaoud in the Paris terrorist operation where they both died.
FBI Director James Comey described this phenomenon as "flash to bang"—the time between radicalisation and crossing the threshold to violent action—which he said can be very short.
Comey last week said "dozens" of suspected radicals in the U.S. have been under "tight surveillance."
"Together we are watching people of concern using all of our lawful tools. We will keep watching them and if we see something we will work to disrupt it," Comey said.
