In the fight against terrorist organizations, one weapon has been effective in the past: cutting off their funding.
Terrorist groups tend to get their money from outside donors or charities.
But the Islamic State, the group that now controls huge areas of Syria and Iraq, doesn't get its money that way.
So the methods the U.S. Treasury has used to fight terrorist groups in the past won't work as well.
Right after the Sept. 11 attacks, the Bush administration created an intelligence unit within the Treasury Department. Its job: to use secret information to track money flowing to terrorist and criminal organizations — and then try to stop it.
"Different targets will require different types of tool sets," says Matthew Levitt, who was the deputy chief of Treasury's office of intelligence and analysis at the time.
"One is trying to stem the flow of funds, drying the swamp. The other is to try to make it so they can't access money, place money in the formal or informal financial system, can't move money," he says. "That's a success as well."
Typically, terrorist groups get donations from overseas; the Islamic State initially got some support from Gulf nations including Kuwait and Qatar.
Those funds arrive through regional banks or the informal hawala system of transferring money.
In the past, Treasury officials have successfully convinced countries to stop donations — or to convince regional banks to crimp those funding streams.
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