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.
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.
Other new details about the raid emerged on Wednesday, including that American forces recovered a number of laptops and cellphones from Mr. al-Baghdadi’s compound, and that, according to the Pentagon, six other people were killed in the compound, aside from Mr. al-Baghdadi and the two children he took with him when he detonated a suicide vest.
Mr. al-Baghdadi spent his last months in an isolated villa in Barisha, a village in a part of Idlib Province dominated by rival jihadist groups and hundreds of miles from the Islamic State’s former territory along the border between Syria and Iraq.
It should have been an inhospitable place for the Islamic State’s leader, much less his final refuge as he became one of the most hunted men in the world. Members of Hurras al Din’s predecessor had killed and been killed by members of ISIS.
The book of receipts, found in Syria by contacts of Asaad Almohammad, a retired American intelligence operative, looks like dozens of others that Islamic State bureaucrats abandoned in the offices they occupied as the administration they once ran crumbled along with their territorial caliphate.
The book contains eight receipts dated from early 2017 to mid-2018 showing payments by ISIS to members of Hurras al Din for security and media equipment, salaries and logistical expenses. The receipts were issued under the logo of the Islamic State’s Ministry of Security and signed by men identified as officials of Hurras al Din.
One receipt issued in the summer of 2018 says it is for $7,000 “to pay for the preparation of bases for the brothers arriving from Al Khair Province,” the name ISIS gave to the region around Deir al-Zour in eastern Syria. The region was one of the last corners of Syria the group’s fighters held before losing it earlier this year.
The receipt suggested that Hurras al Din was helping move ISIS fighters out of the region as they came under pressure from an American-backed, Kurdish-led militia.
Mr. Almohammad, now a senior research fellow at the George Washington University Program on Extremism, said that the receipt also showed that at a time the two groups considered themselves enemies, “ISIS appears to have been trying to infiltrate” Hurras al Din.
The records do not indicate an alliance at the organizational level between the two groups, said Aymenn Jawad al-Tamimi, an independent Syria researcher.
He cited a public statement by Hurras al Din in February that called on its members to avoid contact with members of ISIS, and an announcement by ISIS in its weekly newsletter in April 2018 saying that members of Hurras al Din should be excommunicated.
But if the documents are authentic, he said, they show the existence of a back channel in which cash was transferred between ISIS’s feared security branch and high-level members of Hurras al Din.
He agreed to review the eight receipts and concluded that they did not appear to have been forged, based on the terminology they use and the receipt’s markings, which match other ISIS records.
While the announcement of Mr. al-Baghdadi’s killing on Sunday took many by surprise, there were indications as far back as February that ISIS had not just penetrated its rival’s lair in Idlib Province, near the Turkish border, but was starting to use it as a refuge.
In early February, when ISIS’s caliphate had been cut down to a slip of land the size of Central Park, the American-backed Syrian Democratic Forces temporarily halted their advance and opened a line of communication with ISIS. In the negotiation that followed, ISIS leaders demanded safe passage to Idlib for their remaining fighters, according to commanders briefed on the matter, indicating that they may have already had a foothold there.
The two American officials, who spoke on condition of anonymity to discuss confidential intelligence, said that Mr. al-Baghdadi had arrived at the compound in Idlib by July.
For three and a half months until last weekend’s raid, American intelligence maintained surveillance on the area, initially deeming it too dangerous for Special Forces to enter because of the presence of Qaeda-linked groups and because Russia and the Syrian government controlled the airspace.
After President Trump announced the sudden withdrawal of American troops from northern Syria two weeks ago, the raid was rushed into motion because the Pentagon feared that the United States might lose visibility of Mr. al-Baghdadi.
Although ISIS was paying off members of the Qaeda-linked cell for protection, Mr. al-Baghdadi was ultimately betrayed by one of the few people he trusted, American officials said.
The informer’s identity has not been revealed because of concerns for his safety. But one individual with knowledge of the events described him as someone who is “a very, very close, trusted confidant of Baghdadi’s.”
The informer was recruited by the intelligence arm of the Kurdish-led militia, the Syrian Democratic Forces, which had extensive contacts in the region. The informer was also the one who stole a pair of Mr. al-Baghdadi’s underwear and obtained a blood sample for DNA testing to confirm that the subject under surveillance was Mr. al-Baghdadi, the militia’s commander, Mazlum Abdi, told The New York Times.
Several ISIS members who saw Mr. al-Baghdadi in the last five years said that he had become so paranoid about the possibility of his inner circle being infiltrated that he mostly entrusted his security to a handful of close aides and members of his extended family.
He was one of five sons and several daughters of a Sunni couple from the village of Al Jallam in central Iraq.
Mr. al-Baghdadi’s killing was followed roughly a day later by the killing of Abu Hassan al-Muhajir, the group’s spokesman and the only other well-known public figure inside ISIS, who died in an airstrike near the Syrian town of Jarablus.
American officials, as well as analysts who closely track the group, believe that an associate of Mr. al-Baghdadi’s known as Hajji Abdullah may be named the group’s next leader. The State Department recently announced a $5 million reward for information leading to his capture, describing him as one of the group’s most senior ideologues, who played a direct role in organizing and justifying ISIS’s rape and enslavement of women from the Yazidi minority in Iraq.
ISIS has neither acknowledged Mr. al-Baghdadi’s death nor announced a successor. On Wednesday, the group’s media arm, Al Furqan, which typically publishes messages from ISIS’s leaders, put out a teaser saying that a release was coming “soon.”
As more and more midlevel ISIS leaders have been killed in recent years, analysts described a black hole in terms of understanding its leadership below Mr. al-Baghdadi. American officials said that one of their immediate goals was to begin analyzing the data found in the roughly five to six telephones, two to four laptops and handful of thumb drives found in the compound where Mr. al-Baghdadi was killed.
The data was far less than that retrieved in Abbottabad, Pakistan, during the raid that killed the Qaeda chief Osama bin Laden, said one current official. Still, it could be the most important window into the functioning of the enigmatic caliph and the organization he left behind.
In a news conference on Wednesday, the Pentagon said that six people, including four female Islamic State members, were killed in the raid on Mr. al-Baghdadi’s villa.
Gen. Kenneth McKenzie, the head of the United States Central Command, said the six did not surrender when ordered to do so in Arabic, and that the raiding team feared they might have been wearing suicide vests.
An undetermined number of other fighters outside the compound, who General McKenzie said were not believed to be Islamic State members, fired on the American helicopters. Those fighters were killed in American airstrikes, he said.
He said that Mr. al-Baghdadi took two children — not three as officials had originally said — into a dead-end tunnel with him as American forces bore down. Both children were killed when Mr. al-Baghdadi detonated his suicide vest.
The Pentagon has still not released the name of the military dog who chased Mr. al-Baghdadi into the tunnel, but General McKenzie said the dog was injured by live electrical cables. The dog has since been treated and returned to duty.