FBI Director James Comey says the Islamic State group's online attempts to recruit Americans pose a bigger terror threat to the U.S. than al-Qaida.
Speaking late Wednesday at the annual Aspen Security Forum in Colorado, Comey warned of the extremist group's intensifying social media campaign.
"ISIL is not your parent's al-Qaida," said Comey, using an alternate name for the Islamic State. "ISIL is buzzing on your hip. They're right there on Twitter."
While al-Qaida is careful in how it chooses operatives, he said the Islamic State has cast a wider net and is willing to recruit "unstable, troubled drug users."
Comey said Islamic State-linked Twitter accounts have about 21,000 English-language followers, hundreds or thousands of whom are in the U.S.
The FBI has hundreds of open investigations involving Americans who may have become radicalized and wanted to carry out terror attacks, he said.
He confirmed the bureau had not been tracking Muhammad Abdulazeez, identified as the gunman who shot and killed five service members last week at military sites in Tennessee.
Investigators have not determined whether Abdulazeez was inspired by Islamic militants. His family has said he suffered from depression and used drugs.
Comey says he fears more attacks, thanks to the Islamic State's campaign urging Muslims to "kill where you are" if they cannot travel to the Middle East. The recruitment campaign has been especially "blossoming in the last six to eight weeks," he said.
He said the Islamic State initially corresponds with recruits via Direct Message on Twitter before moving to more secure, encrypted communications, said Comey.
Encrypted messaging services have become more commercially available in recent years, posing a unique challenge to counter-terror investigations.
U.S. intelligence officials have been pushing for expanded "backdoor" access to encrypted communications, over the objections of privacy advocates.