Is ISIS the world's wealthiest terror group?

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.

Maps: Crisis in Iraq

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.

#ISIS is richer than some countries. Here are 6 major ways the militant group makes its money: http://t.co/XBXdyK8lZB pic.twitter.com/oFMMbHVnxa

— CNN International (@cnni) June 23, 2014