The terror group known as ISIS, ISIL, or Daesh, that self-identifies as the Islamic State, is apparently raking in big bucks by gaming financial systems in the Middle East.

Experts on the group last month told a subcommittee of the United Kingdom Parliament that ISIS has been exploiting loopholes in foreign exchange operations, CNBC reports. The militant group could be making as much as $25 million per month, the experts said, on top of what it is already making from taxes, extortion, donations, smuggling migrants, and trafficking in oil.

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David Butter, an associate fellow at Chatham House, an international affairs think-tank, argued before the Foreign Affairs committee that common estimates of the group’s oil revenues—more than 40% of a reported total of $80 million a month—are over-inflated. (He also argued that “monthly” metrics are misleading, given the instability of the group’s finances.) Rather, he and others on the panel said, a more reliable recurring source of revenue for ISIS involves shuffling funds across borders, taking advantage of exchange rate fluctuations and an informal network of brokers called “Hawala.”

“There are other places you need to look for the money,” Butter told the subcommittee, which was tasked with exploring ways to disrupt the terror group’s finances. (You can watch the full video here.) “The Iraqi central bank foreign currency auction systems are an area that needs to be investigated very strongly.”

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ISIS members, Butter said, have moved money from banks in Mosul, an ISIS-controlled city in northern Iraq, to ones in Jordan. Then they’ve moved it back into Iraq through Ramadi, formerly an ISIS bastion in central Iraq. The complex arrangements allow the group to exploit imbalances in currency markets. “So when the Iraqi Government does its regular foreign currency auctions, the ISIS money is inserted into that system and they can make a margin on the differences between the various exchange rates there and send it back into their areas through Hawala operatives,” Butter said. “This is the way money moves in the Middle East.”

As far as solutions to impeding ISIS funding go, Butter suggested that the international community should work closely and cooperatively with Iraqi financial authorities in order to close gaps in the system. “There’s not a magic wand we can throw at this,” he said.