A member of the board of governors of the Central Bank of Iraq was reluctant to specify how much ISIS got away with in Mosul, but estimated that it at least $85 million and possibly much more. He spoke on the condition of anonymity because he was not authorized to speak publicly about the case.

“ISIS gets some money from outside donors, but that pales in comparison to their self-funding,” said an American counterterrorism official. “The overwhelming majority of its money comes from criminal activities like extortion, kidnapping, robberies and smuggling. In Mosul, ISIS has probably been hauling in several million dollars monthly just from its extortion racket. In overrunning the town the group is better off financially, but probably to the tune of millions — not hundreds of millions — of dollars.”

While ISIS “is among the wealthiest terrorist groups on the planet,” the official said, “it also has significant expenses. Resources flowing into the group’s coffers tend to move out the door in the form of payments fairly quickly. Unless it has invested very wisely, it’s probably sitting on a pile of assets worth somewhere in the tens of millions of dollars.”

The militant group has so much cash that it has reopened some of the banks it looted in Falluja, in Anbar Province, to stash it in. Jassim Ahmed, 35, who works as a taxi driver in the city, which has been under militant control since January, said he asked one of the gunmen guarding the banks where the militants get their money. “Don’t ask me again,” the gunman told him, he said. “Just understand, we have a budget to administer all of Iraq, not just Anbar.”

Kamel Wazne, an analyst based in Beirut, Lebanon, who has followed the group’s development into a self-financing, territory-controlling entity, said: “We no longer have to imagine a terror state. We have one.”

ISIS started amassing a bankroll in Syria last year after it took over the eastern Syrian oil fields, near Raqqa. It operates primitive refineries to make products for local use by ISIS’s own fighters, but sells much of the crude to its enemy — the Syrian government. In Minbij, it runs a local cement factory, and in Raqqa merchants even pay the militants a trash collection fee.