Three years ago, the Islamic State of Iraq and Syria, better known as ISIS, was just a small group of extremist Sunni Muslim militants battling to bring down the Syrian government. But in recent weeks, ISIS has emerged as a major insurgency, expanding its Syrian territories and capturing a broad swath of Iraq, including the country's second largest city, Mosul. And this raises an important question: How did ISIS grow so swiftly and raise enough money to buy weapons for its army?

Much evidence suggests that ISIS cashed in on the Syrian oil fields it captured. But two weeks ago, Iraqi intelligence officers discovered new sources of its income, according to a report in the Guardian newspaper. While securing