Islamic State militants, who once relied on wealthy Persian Gulf donors for money, have become a self-sustaining financial juggernaut, earning more than $3 million a day from oil smuggling, human trafficking, theft and extortion, according to U.S. intelligence officials and private experts.

The extremist group's resources exceed that "of any other terrorist group in history," said a U.S. intelligence official who, like others interviewed, spoke on condition of anonymity to discuss classified assessments. Such riches are one reason that American officials are so concerned about the group even while acknowledging they have no evidence it is plotting attacks against the United States.