Former CIA Deputy Director Michael Morell said Tuesday Islamic State terrorists will soon attack America if their rapid expansion is not stopped.

"The head of the UK's domestic security agency recently warned that ISIS is planning mass casualty attacks in Britain," Morell wrote in a Time op-ed Tuesday. "His concerns are well founded. We will not be far behind."

Morell, who served as CIA acting director twice, noted the FBI has already opened more than 900 investigations into "homegrown extremists" that have been radicalized by the Islamic State in the U.S., many of which had been planning local attacks.

While the sophistication of such attacks will likely be low, Morell said the terrorist group could potentially launch a large number of them given its massive group of followers around the world.

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