In the gruesome video showing James Foley's murder, his executioner speaks with a British accent.
To recruit more Muslims from the West, ISIS released a propaganda video in June featuring three British men who encouraged others to join them and sacrifice their lives for Islam.
"Look around you where you sit in comfort and ask yourself 'Is this how you want to die?'" says one of the men in the video.
In a violent rampage beginning two months ago, ISIS extended its territory from Syria into Northern Iraq.
It rules with brutality and fear, publicly executing those who offend its strict version of Islamic law.
What makes ISIS unusual as a terror group is the ambition of its leaders. In the land they control, they've set up what they call an Islamic state and now they're busy making the money they need to fund it.
A video from Syria shows an ISIS fighter policing a local market.
The group levies taxes and even sells gasoline and electricity.
U.S. officials say ISIS is now self-sustaining and is probably making tens of millions of dollars with criminal activities like extortion and smuggling.
In a Turkish town just across the border from Syria, we met Dilal Omarali, who's fought against the Islamic militants in Syria's chaotic civil war.
"They have more money than we do," he told us in Arabic, "and they also have better guns, more tanks, and more missiles."
That's part of the attraction for some young Muslims. And they've found joining ISIS is simple.
We watched in December as Syrian refugees crossed into Turkey by climbing through a wire border fence.
Three men, who locals told us were fighters, slipped past us and into the war zone.
Intelligence officials worry that young European Muslims who have been recruited by ISIS will be ticking time bombs when they return.
They could also pose a threat to America, because those who carry British and some other European passports can enter the U.S. without a visa.