The Islamic State has diverse sources of funding: oil revenues, taxes levied on the people in the territories it now controls, sales of looted antiquities, ransoms paid for kidnap victims. It is a sizable and troubling pool of money.

But experts differ on whether or not it will last.

“In the long term, much of their financial resources are not renewable,” said Matthew Levitt, director of the Stein Program on Counterterrorism and Intelligence at the Washington Institute for Near East Policy. “When they dig up antiquities, there’s no more antiquities. When they sell oil, they don’t have the ability to maintain the upkeep of these wells.”

Plus, he pointed out, the United States is targeting networks ISIS sells to and bombing their wells or conveys. 

Howard Shatz, a senior economist with the Rand Corporation, says the question of whether ISIS’s funding is the sustainable is the new question many clients are asking him, both governmental agencies and foundations. 

His answer? “My own sense from researching the organization for many years is that it is pretty sustainable,” Shatz said. “Its finances are pretty strong and it can stay active.”

But, he cautioned, “no one really knows” how much money ISIS actually has or how long it may last.