Since last month, U.S. warplanes have struck the Islamic State group's oil infrastructure in Syria in a stepped-up campaign of economic warfare that the United States estimates has cut the group's black-market earnings from oil by about a third.
In finding their targets, U.S. military planners have relied in part on an unconventional source of intelligence: access to banking records that provide insight into which refineries and oil pumps are generating cash for the extremist group, current and former officials say.
The intent is to choke off the Islamic State's funding by tracking its ties to the global financial system. By identifying money flowing to and from the group, U.S. officials have been able to get a glimpse into how its black-market economy operates, people with knowledge of the effort have said.