Banks will find it difficult to identify the proceeds of illicit crude oil sales linked to Islamic State, the blacklisted terrorist organization now controlling a handful of Iraqi and Syrian oil fields. The group, which has operated under multiples names since its inception in 1999, is funding itself in part from "extortion-derived proceeds from Iraqi and Syrian oil resources," according to a U.S. Treasury Department spokesperson. The organization's sale of captured crude oil to local traders purportedly tops at least $1 million per day, by some estimates. But where that money goes and who ultimately handles it remains unclear. At...