Editors’ Note: After this article was published, questions were raised about the authenticity of the documents upon which it was based.

The Islamic State leader Abu Bakr al-Baghdadi was able to hide out in an unlikely part of Syria, the base of a rival group, because he was paying protection money to its members, according to receipts for the payments recovered by researchers.

The receipts, typical of the Islamic State’s meticulous bookkeeping, showed that the group paid at least $67,000 to members of Hurras al Din, an unofficial affiliate of Al Qaeda and an enemy of the Islamic State.

[Update: ISIS confirms al-Baghdadi’s death and names new leader.]

While the rival group kept Mr. al-Baghdadi’s secret, he was ultimately betrayed by a close confidant, two American officials said Wednesday, leading to his death in an American Special Forces raid last weekend.