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(CNN) – ISIS has gained a reputation as not only one of the world's most ruthless terror groups, but also one of the wealthiest.
The terror group is so rich, it now has plans to create its own money. ISIS announced on social media that it wants to mint its own gold, silver and copper. Doing so, they say, would free them of any ties to the "tyrants' financial system," a reference to western money, particularly the U.S. dollar.
Al Qaeda, the predecessor to ISIS, principally raised money through external sources, such as donations from Saudi Arabia and even charities. But ISIS is primarily self-funded, says a former under secretary for the Treasury Department.
The group's largest stream of funding comes from illicit oil sales, from wells and refineries controlled by ISIS in Iraq and Syria.
"The estimates are they bring in as much as $1 (million) to $2 million dollars a day from the sale of oil on the black market," said Jimmy Gurulé, now a law professor at Notre Dame.
The group has also raised about $20 million from ransom hostage payments, extortionate payments of taxes imposed on people and businesses in territories ISIS controls, and the sale of ancient artifacts stolen from Iraq and Syria, he said.
Gurulé testified before members of Congress Thursday about stopping the flow of cash into ISIS.
"The Treasury Department needs to be focusing on the middle men, the smugglers, the people that are actually as much as 20,000 to 30,000 barrels of oil a day across the border into Turkey and also into Syria," Gurulé tells CNN.
To date, the department has not placed any of the smugglers or middle men on a list that would block and freeze their assets.
"The Treasury Department has had some difficulty in developing some traction, establishing its footing, and focusing and identifying those individuals," said Gurulé, who was under secretary for the department under then-President George W. Bush.
ISIS is estimated to have as much as $1 million to $2 billion in reserve, said Gurulé. "This is the wealthiest terrorist organization the world has ever known."
Such vast amounts of money has to eventually enter the financial system, he predicts.
"The Treasury Department needs to be focusing on banks, banks in Qatar for example, and in Kuwait, that may be the recipients for handling money for ISIS," said Gurulé.
Even with all the money ISIS possesses, the current Treasury Department Under Secretary David Cohen has said that the group does not have the financial capability to run a government or set up the so-called caliphate that it aspires to establish.
"They certainly have ample funds to finance their terrorist operations, to kill innocent civilians, to orchestrate, and plan, and implement terrorist attacks," he said. "They shouldn't be taken lightly."