Sources tell CNN Iraq's largest oil refinery is now believed to be in the hands of ISIS – the Islamic State in Iraq and Syria.
They have overrun Mosul, Iraq's second-largest city. In recent weeks, they've wrested control of Iraqi cities like Falluja and parts of Ramadi from authorities, just as they've done with Syrian towns over the border.
ISIS has been called the world's wealthiest terror group, with more resources than even al Qaeda.
CNN'S Michael Holmes looks into how they are bankrolling operations.
â€” CNN International (@cnni) June 23, 2014