Fox news chief intelligence correspondent Catherine Herridge appeared on Happening Now today from Washington to discuss Director of the National Counterterrorism Center Matt Olsen's assessment of ISIS and what threat it poses to the U.S. homeland.

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The terror group controls an area the size of Great Britain, commands 10,000 armed fighters and is taking in one million dollars a day from oil sales and ransoms, according to Olsen during an intelligence forum yesterday.

"Left unchecked, they will seek to carry out attacks closer to home. But no terrorist group, certainly not ISIS, is invincible," Olsen said.

Olsen noted ISIS's use of social media, including the beheadings of American journalists James Foley and Steven Sotloff, which sets them apart as the most successful terror organization at spreading propaganda globally.

Olsen said that while ISIS is indeed a direct threat to U.S. buildings and personnel in Iraq, and the group has the ability to launch small-scale attacks in neighboring countries, the current focus is on whether the 100 American-born ISIS fighters with U.S. passports will return to attack the homeland.

Although Olsen said there is no indication of terror cells in the U.S. or any planned attacks, Herridge pointed out that there is already evidence that an American al-Qaeda recruit traveled to Syria, returned to his home state of Florida and then flew back to Syria in May to launch a suicide attack against Syrian government officials, with the FBI disturbingly unaware of his movements along the way.

Watch the clip from Happening Now above.

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