WASHINGTON, D.C. — Battled-hardened “holy warriors” who traveled from outside the Middle East to join the Islamic State (ISIS/ISIL) in Iraq and Syria are expected to “wreak murderous havoc” in the United States and other countries once they return home, warns the secretary of the U.S. Department of Homeland Security (DHS).

According to written remarks prepared for an address he delivered Tuesday on the DHS response to threats against United States, DHS Secretary John Kelly said:

Experts estimate that perhaps 10,000 citizens of Europe have joined the [ISIS] caliphate in Syria and Iraq. Thousands more are from nations in Asia, Africa and the Western Hemisphere. They have learned how to make IEDs [improvised explosive devices or homemade bombs], employ drones to drop ordnance, and acquired experience on the battlefield that by all reports they are bringing back home. As the coalition we lead wins against what are best described as conventional-terrorist formations in the caliphate [in Iraq and Syria], the expectation is that many of these ‘holy warriors’ will survive departing for their home countries to wreak murderous havoc in Europe, Asia, the Maghreb, the Caribbean and the United States. And because many are citizens of countries in our Visa Waiver Program [VWP], they can more easily travel to the United States which makes us a prime target for their exported violence.”

VWP allows citizens of 38 countries, the majority of them in Europe, to use their passport to travel to the U.S. without a visa for stays of up to 90 days.

Retired Gen. Kelly delivered his address at George Washington University (GWU).

The return of the ISIS foreign fighters to the United States may further fuel the already “unprecedented” increase in homegrown terrorism facing the nation.

Kelly revealed that, in the past 12 months alone, the number of homegrown terrorism cases reached 36 in 18 U.S. states.

“Over the past few years, we’ve seen an unprecedented spike in homegrown terrorism,” he pointed out.

“These are the cases we know about—homegrown terrorism is notoriously difficult to predict and control,” also noted Kelly. “And what’s feeding this homegrown violence? Most experts agree a major contributor is the internet.”

Moreover, the DHS secretary noted that the FBI had opened terrorism investigations across all 50 U.S. states since 2013. Among the cases are at least 37 ISIS-linked plots to attack the American homeland.

Kelly said the threat environment in the United States, which primarily involves Islamic terrorism and transnational criminal organizations (TCOs) such as drug cartels and violent gangs, has worsened since the 9/11 attacks.

“The threat to our nation and our American way of life has not diminished. 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,” proclaimed Secretary Kelly.

“We are under attack from terrorists both within and outside of our borders. They are without conscience, and they operate without rules. They despise the United States, because we are a nation of rights, laws, and freedoms. They have a single mission, and that is our destruction,” said Kelly.

“I tell you, without exaggeration, they try to carry out this mission each and every single day,” he continued.