In a very unusual airstrike, the U.S. dropped bombs on Sunday in central Mosul, Iraq, destroying a building containing huge amounts of cash ISIS was using to pay its troops and for ongoing operations, two U.S. defense officials told CNN.

Officials could not say exactly how much money was there or in what currency, but one described it as “millions.”

Since November, U.S. and Coalition forces have been trying to kill ISIS by going after their main money source — oil smuggling. They have been bombing ISIS-held oil fields and targeting tanker trucks carrying the black gold to Turkey.

In July, NBC News reported that the Islamic fanatics were making three times as much from oil smuggling as U.S. officials previously thought, roughly $8 million to $10 million a month.

The new intelligence about ISIS finances was discovered after U.S. commandos killed the militants’ “money man”, known as Abu Sayyaf, last May in a raid on his Syrian hideout. They also captured his Iraqi wife, identified as Umm Sayyaf, and seized several computers from their compound.

ISIS uses some of the blood money to pay its fighters monthly salaries and provide stipends to their families. Foreign fighters, who are the highest paid recruits in the ISIS ranks, earn as much as a $1,000 a month, two Syrian sources told NBC.

Former ISIS fighters and Syrians and Iraqis within ISIS-held territory say the group’s zakat tax, which is a form of almsgiving in Islam, is keeping its economy churning.

Zakat, which dates back to the days of the Prophet Muhammed, requires Muslims to hand over portions of their income and can be given to those who are fighting for a holy cause, the Financial Times reported.

As Zakat, ISIS takes 2.5 percent of capital from wealthy residents and businesses, 5 percent of irrigated crops and 10 percent of rain-fed crops from farmers, and days of service from doctors, who are forced to volunteer once a week at area hospitals. The result is hundreds of millions of dollars in revenue each year, the investigation found.

ISIS even collected $23 million in taxes on salaries the government of Iraq paid to officials in the city of Mosul for almost a year after it was taken over by the group.

Estimates based on statistics provided by Iraqi officials and Syrian farmers reveal that $20 million worth of zakat was collected on grain and cotton, while trucks traveling into ISIS territory were charged customs duties totaling up to about $140 million a year, the FT report said.