According to FBI Director James Comey, U.S. military strikes have diminished the threat posed by the al Qaeda offshoot Khorasan Group, but the Islamic State still poses a significant threat.
"The threat that ISIL presents and poses to the United States is very different in kind, in type and degree than al Qaeda," Comey told CNN's Wolf Blitzer. "ISIL is not your parents' al Qaeda. It's a very different model. And by virtue of that model, it's currently the threat we are worried about in the homeland most of all."
Comey also compared tracking Islamic State militants to finding needles in a haystack.
"If you imagine a nationwide haystack, we are trying to find needles in that haystack. And a lot of those needles are invisible to us either because they are communicated or just because they have communicated in a place that we can't see them," he said from Aspen, Colo. "And knowing there are needles out there that you can't see is very worrisome."
Comey said the possibility of a terror group launching a cyber attack on the U.S. is small but growing.
The U.S. has been leading coalition airstrikes in Iraq and Syria against terror groups like the Islamic State since summer 2014.