Ransom payments are one of ISIS’s major sources of income, with “tens of millions of dollars” paid by some European governments and wealthy relatives of the kidnap victims over the past two years. The low end of the estimate range is “well above $25 million,” according to the official, who spoke on condition of anonymity.

In addition to collecting ransom, ISIS feeds its ravenous hunger for cash primarily through extortion and oil smuggling, said the official.

But while those activities are “the bread and butter,” no revenue stream is considered too insignificant, added a second official, noting that ISIS “requires drivers to pay ‘road taxes’ in territories it controls.”