The Islamic State group, possibly the richest terrorist entity on the planet, has a diversified financial strategy that includes everything from oil sales to kidnapping and antiquities smuggling, which funds their barbaric caliphate ambitions and salary payments.

Once they get the cash it has to go somewhere. And it’s probably not a bank.

“Instead of a banker you have a hawalador,” Colin Clarke, associate political scientist and threat finance expert at RAND Corporation, told International Business Times.

Hawala is a method of value transfer system that relies on a network of brokers to send money to various locations instantaneously and without a paper trail. The ancient system used especially in the Muslim world is convenient for its users, but difficult for those looking to track them down.

Basically, a person can transfer money from one location to another by depositing money with a local agent, who calls their counterpart abroad. Then, the recipient will collect their cash from this second person instantly and the two agents settle their accounts later.

“It’s mostly secure, it’s relatively anonymous and it’s also convenient for those who don’t want to deal with the scrutiny or due diligence procedures of banks,” Clarke said.

The networks tend to be long-established, based on family relationships or regional affiliations, and broker reputations are built mainly on trust. While this method is sometimes referred to as “underground,” it’s often used for legitimate reasons and even advertised, according to a report from the U.S. Treasury.

“Regular people use this. That’s what makes it so hard to track; it would be a lot easier if it was only dirty money,” Clarke said.

Investigators have been moving to regulate this system for more than a decade. The U.S. government identified hawala when it was a primary method used by al Qaeda to send and receive cash. Just one month after 9/11, it was named in a report from the Financial Action Task Force, an international organization focused on fighting money laundering and terrorism financing, calling for increased transparency in global money networks.

Of course, this isn’t the only way Islamic Group is moving money. There are also less complex methods that are even harder to track.

“You can literally drive a car with $10,000, $20,000 or a million dollars from XYZ country to Syria,” a former U.S. counter-terrorism official told the L.A. Times. “Not a whole lot we can do.”