Story highlights The FBI currently has open terrorism investigations in all 50 states

Fighters from Asia and Europe who have been trained by ISIS are bringing those skills home

Washington (CNN) Department of Homeland Security Secretary John Kelly issued a stark warning Tuesday, calling the risk of a terror attack on the United States "as threatening today" as it was on 9/11.

Specifically, Kelly outlined the danger posed by homegrown terrorists and foreign fighters from around the world who have traveled to Syria and Iraq with plans to return home.

"The threat to our nation and our American way of life has not diminished," Kelly said during a speech at George Washington University. "In fact, the threat has metastasized and decentralized, and the risk is as threatening today as it was that September morning almost 16 years ago."

According to Kelly, the FBI currently has open terrorism investigations in all 50 states, and since 2013, there have been 37 ISIS-linked plots to attack the US.

Law enforcement agencies have investigated 36 cases of homegrown terrorism over the last 12 months, illustrating what Kelly called an "unprecedented spike" in a type of violence that is "notoriously difficult to predict and control."

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