U.S. authorities say they have arrested an American man returning from overseas who is an alleged sympathizer of the terrorist group ISIS.

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Donald Ray Morgan was arrested on August 2 on arrival at New York City's John F. Kennedy International Airport and charged with illegal weapons possession.

Rep. Peter King (R-New York) joined Martha MacCallum on America's Newsroom today and said that he doesn't think Morgan was actually working with ISIS, but more likely was inspired by them and "self-radicalized."

Although King gave the FBI credit for catching Morgan, he said that other ISIS sympathizers from the U.S. and Europe will be much more difficult to track and represent a growing concern. He pointed to reports of more than 100 Americans currently fighting in the civil war in Syria.

He noted that the intelligence communities and various agencies are coordinating their efforts to combat this rising threat, in addition to working closely with other countries, particularly our European allies.

Watch the clip above.

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