James Comey said the threat from Al-Qaeda, particularly its Khorasan affiliate in Syria, has been "significantly diminished" by U.S. air strikes that just this week killed veteran operative Mushin al-Fadhi, according to RFE/RL.

IS, by contrast, has grown increasingly effective at recruiting impressionable, "troubled souls" through social media, urging Muslims who can't travel to the Middle East to "kill where you are."

He said IS has 21,000 followers on Twitter worldwide.

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The FBI has arrested a significant number of people recently who were radicalized, likely preventing attacks, but hundreds more suspects remain at large, he said.

Comey noted that "the people the Islamic State is trying to reach are people that Al-Qaeda would never use, because they are often unstable, troubled drug users."

Only a year ago, the FBI largely dismissed the threat from IS and was more worried about a mass casualty attack against an airliner by Khorasan or Al-Qaeda's Yemen affiliate.