On Monday, U.S. A-10 attack planes and AC-130 gunships attacked a fleet of trucks the Islamic State has been using to smuggle oil out of Syria, a major source of income for the militant group. According to initial assessments, the U.S. aircraft destroyed 116 of the 295 trucks at the facility near Deir al-Zour, Syria. The U.S. had refrained from attacking the tankers earlier out of concerns about civilian casualties, The New York Times reports, and U.S. jets dropped leaflets an hour before the bombing telling drivers to abandon their trucks, among other actions designed to spare civilian lives. ISIS used its fleet of about 1,000 tanker trucks to earn up to $40 million a month to fund its operations. The operation was planned before the ISIS-linked terrorist attacks in Paris on Friday.