Washington -- For the head of the FBI's counterterrorist division, Michael Steinbach, the unknown worries him the most.
Steinbach is leading the daunting effort to stay on top of the evolving threat landscape, which includes targeting and recruiting teenage Americans. In an exclusive interview with CNN inside the agency's Strategic Information and Operations Center, he acknowledged it's extremely difficult to track every American who might travel abroad to join terrorist groups like the Islamic State.
"I'm worried about individuals that we don't know about that have training," Steinbach said. "We know what we know. But there is a number that's greater than that that we don't know."
Steinbach says U.S. law enforcement and intelligence agencies don't track individuals leaving the United States to vacation in Europe.
"Once you get to Europe, you can easily get down to Turkey and into Syria" Steinbach says.
There's growing concern about homegrown violent extremism in the aftermath of last month's terror attacks in Paris. Those strikes underscored the threat posed to the West by small groups of terrorists with western passports who are influenced by the rhetoric espoused by ISIS. Steinbach is concerned that type of attack could happen on U.S. soil.
When asked if there are ISIS cells in the U.S., Steinbach said "there are individuals that have been in communication with groups like ISIL who have a desire to conduct an attack" and those people are living in the U.S. right now, but he says the term "sleeper cells" is too simplistic, because the threat is much more complicated and diffuse.
In the U.S., the FBI has seen children as young as 15 recruited by ISIS and Steinbach said he "can't speak with 100% certainty that individuals of that age group have not gotten over there successfully."
In some cases, Steinbach said parents even encourage their children to be involved with terror groups.
"There are individuals out there who are inspired by the message of terrorist groups and they encourage family members, including their children, to follow that path," he said, adding in those cases, the FBI holds the parents responsible.
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Steinbach said the FBI is working around the clock to combat the recruitment of Americans but U.S. law enforcement can't do the job alone. It's up to families to speak up as well, he said.
"In the majority of cases, we know that someone recognizes that change in behavior, that radicalization," he said. "That family member or friend chooses not to intervene. And by not getting involved, the story ends in a very familiar fashion, and that's death."
He also said ISIS is aggressively pursuing women on social media.
"The recruitment of women by ISIS is much more than we've ever seen by a terrorist organization," he said. "We have seen everything from a female fighter -- dedicated groups of women fighters -- and those who have come over to support foreign fighters by marrying them."
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He emphasizes ISIS is pushing out a false narrative of what it's like in Syria in order to lure them.
Monitoring social media poses its own challenges, he said. The sheer volume of posts calls for strong analytical skills to weed through the data, which he said is a "full time job and a challenge."
"We've seen lots of places, online media, forums, social media, where there have been calls to conducting lone wolf attacks in your home country through a variety of means, not necessarily a sophisticated technique, but use what you have, use the tools you have and conduct an attack," Steinbach said. "They are using it successfully, I might add, to spot, assess, identify, target folks outside of war zones," he elaborated.
And, of course, the FBI must also strike a delicate balance of respecting privacy concerns of Americans while trying to protect them.
"We don't have a desire or a right to step on somebody's freedom of expression. They have a right to express their opinion," Steinbach says. "But when that opinion turns into violent rhetoric and then into action, that's something different."
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Still, the recent arrest of an Ohio man, Christopher Lee Cornell, has drawn criticism that the government is making terrorists out of people. He raised red flags by posting messages supportive of violent jihad on social media and was eventually arrested for plotting to attack the U.S. Capitol. But there were doubts about how seriously Cornell sought to act on his threats.
To his critics, Steinbach says, "I need folks to understand that whether you're talking about a foreign terrorist organization directing individuals or just inspiring individuals...we identify individuals with the intent. We don't manufacture that intent. We don't put that intent into their mind."
But do they have the capability and manpower to combat so many individuals with the intent of attacking Americans?
"I don't know if enough manpower is the right word," he said. "Look, there are lots of threats out there, criminal threats, counter-intelligence-based threats, cyber threats and terrorism threats. And we have to identify those highest priority threats and focus the resources. There is a finite number of resources and we have to focus those resources on those threats."
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