The FBI followed "dozens and dozens" of potential jihadists across the United States over the summer and "disrupted" many of them, its director James Comey has told a Senate hearing.
Encouraged by extremist groups such as Islamic State (IS), the number of home grown radicals has jumped dramatically in the past 18 months, the Homeland Security and Governmental Affairs Committee was told on Thursday (local time).
Comey admitted that hunting down potential militants was like "finding those needles in a nationwide haystack".
"This summer we were following dozens and dozens of people, all over the United States, 24/7," Comey said.
"We disrupted a lot of those people."
The social media campaigns of extremist groups like IS had found an increasingly receptive audience in the US, he warned.
"It seems to be drifting younger, with more girls – by girls, I mean women under the age of 18 – with whom this message on social media is resonating."
Nicholas Rasmussen, director of the National Counterterrorism Center, said the number of home grown Islamists had surged.
"If you go back to 2009, we were seeing on average less than two or three of these incidents a year, Rasmussen said.
"By last year, the number rose to a dozen and today this number has already doubled," he said of instances of home grown threats identified by US authorities.
"There's no question in my mind this population has increased in size dramatically over the last 18 months.
"ISIL (IS) has injected new energy in life of the population of home grown violent extremists."