Matt Olsen, a former Obama administration intelligence official, says the United States will likely see a homegrown terrorist attack in the next year or two.

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"If you look out in a 12- to 24-month time frame, I think it’s safe to say that there is a very significant chance that we’ll see at least one or more small scale terrorism attacks in the United States like we’ve seen in Europe."

Olsen, who served as Director of the National Counterterrorism Center during the Obama administration, attributed the limited number of attacks on U.S. soil to sophisticated intelligence gathering efforts and geographical location. But, he says it will be hard to stop an attack inspired by the Islamic State in Iraq and Syria's aggressive methods in recruiting foreign fighters online.