There is no evidence a married couple who killed 14 people in California this month were part of a terrorist cell, the head of the FBI says.
The Islamist militant group has "revolutionised" terrorism by seeking to inspire such small-scale attacks, FBI Director James Comey said on Wednesday, noting the group uses social media, encrypted communications and slickly produced propaganda to recruit followers around the world.
"Your parents' al-Qaeda was a very different model than the threat we face today," Comey told a counterterrorism conference in New York.
However, he said that while the perpetrators of the December 2 shootings in San Bernardino, California - Syed Rizwan Farook, 28, and Tashfeen Malik, 29 - had expressed support for "jihad and martyrdom" in private communications as early as 2013, they never did so publicly on social media.
He also said authorities believe Mohammed Abdulazeez, the suspect in July's fatal shooting of four US Marines and a Navy sailor in Chattanooga, Tennessee, was radicalised by militant propaganda.
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"To my mind, there's no doubt that the Chattanooga killer was inspired and motivated by foreign terrorist organisation propaganda," he said, but did not specify any particular group.
Comey said the Federal Bureau of Investigation currently has "hundreds" of investigations in all 50 US states involving potential Islamic State-inspired plots.
Islamic State controls wide swaths of Iraq and Syria, where it seeks to carve out a caliphate.
The group has a three-pronged strategy, Comey said: recruit fighters to join it in the Middle East; inspire individuals in other countries to carry out attacks; and send out trained operatives to commit violence in Europe and the United States.
