Western intelligence officials say looting is now Islamic State’s second-largest source of finance after oil. But can that figure be calculated?

Experts say trade in stolen antiquities plays a central role financing the militant group, also known as ISIS, even if they aren’t able to calculate the value of revenue sourced this way.

“What we know is that sites under ISIS control are being looted on a massive scale,” said Patty Gerstenblith, a professor of cultural heritage at DePaul University College of Law in Chicago. Syrians on the ground say that Islamic State charges a tax or regulates the middlemen who purchase these artifacts, she said.