American photojournalist James Foley was beheaded by a member of the Islamic State, his death shown in a YouTube video released on Tuesday. Foley’s executioner spoke fluent English with a strong East London accent, leading experts to believe that the jihadist is a British national.

British foreign secretary Philip Hammond stated in an interview with BBC Wednesday that leaders are “absolutely aware that there are significant numbers of British nationals involved in terrible crimes, probably in the commission of atrocities, making jihad with ISIL [now known as the Islamic State] and other extremist organizations.”

It is estimated that dozens of Americans and thousands of Europeans are currently fighting under the flag of the Islamic State.

The release of this video has highlighted the existence foreign fighters in Iraq and Syria, and has made cutting off their pipelines a top security concern. Young men are being targeted and recruited by extremist groups like the Islamic State, and are being funneled into Iraq and Syria.

These pipelines are taking advantage of porous borders and limited security in war-torn countries to move thousands of foreign fighters between conflict zones and western nations. It is estimated that dozens of Americans and thousands of Europeans are currently fighting under the flag of the Islamic State. As the process of recruiting, training, and transporting foreign fighters becomes more systematic, the threat to the United States and her allies increases.

Foley’s death highlights the barbarism of the Islamic State, and the threatening role that foreign fighters are playing. So far, the U.K prime minister David Cameron has responded by returning from his vacation early to take meetings on the situation in Iraq and Syria. On Wednesday afternoon, President Obama denounced the actions of the Islamic State and affirmed that the U.S. would continue to conduct airstrikes to “do what we must to protect our people.”