Federal law allows for the prosecution of minors as adults – in some cases as young as 13 -- if they are accused of a violent offense, using a firearm in committing a violent offense or felony drug trafficking or importation. But minors who do not meet those criteria – including the known cases involving minors intending to join ISIS – are only subject to a lesser charge of juvenile delinquency. If charged with that, they must be released from custody at age 18.
There have been other recent indications that ISIS is expanding its recruitment efforts beyond prospective male fighters.
Read More Tufts University renames student information portal because of ISIS
Intelligence officials, speaking on condition of anonymity, tell NBC News that "dozens" of British women have joined ISIS in the last two years.
Meantime, the news website Vocative reported that ISIS had launched an online propaganda campaign specifically aimed at women last month. Called "al-Zawra," the campaign uses social media platforms and videos to disseminate a "how-to" of jihad for young women. Aside from offering pointers on such skills as cooking and sewing, the campaign aims to "to teach sisters software design and editing," and "the physical side and ... the use of weapons," it said.
And three months ago, ISIS released a recruiting video showing a young girl, flanked by two gun-toting adult male fighters. In it, the girl, who appears no older than 10, calls on children to join the group and fight against the enemies of Islam, including Shiites and Jews. The same video showed young boys involved in military training.
Read MoreHere's your hour-by-hour guide to Tuesday's elections
Comey said the new thrust of ISIS' recruitment efforts adds to the challenge of identifying potential jihadis and intercepting them before they can join the fray.
"The challenge of the traveler phenomenon is there's no typical profile," he said at the Fordham event, noting that the FBI has charged individuals aged from 18 to 63. "As best we can determine, the common feature is they are troubled souls who are seeking some kind of meaning in their life. ... Finding a typical person, a physical being in which that troubled mind sits is actually something that's eluded us."