The Islamist militant group has 'revolutionized' terrorism by seeking to inspire such small-scale attacks, FBI Director James Comey said, noting the group uses social media, encrypted communications and slickly produced propaganda to recruit followers around the world.

However, he said that while the perpetrators of the Dec 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 U.S. Marines and a Navy sailor in Chattanooga, Tennessee, was radicalized by militant propaganda.

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 also frequently employs encrypted communications, Comey said. He renewed his calls for technology companies to avoid creating devices and services that cannot be accessed, even with a proper court order.

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