A member of the inner circle of the Islamic State's leader, Abu Bakr al-Baghdadi, was key to helping US commandos execute the raid on al-Baghdadi's compound on Saturday, The Washington Post reports.

The informant was a disillusioned Islamic State member who helped oversee construction of al-Baghdadi's hideout in Syria's Idlib province. The Post said that the informant was present during the raid, in which al-Baghdadi was killed, but that he and his family had since been removed from the area.

The informant's nationality is unknown, but he has been assisting the US and partner forces for months, retrieving a pair of al-Baghdadi's used underwear three months ago to confirm the Islamic State leader's presence in the compound in Idlib, The Post said.

Visit Business Insider's home page for more stories.

A trusted confidant of Abu Bakr al-Baghdadi turned against the Islamic State leader, playing a key role in the US raid Saturday night that ended in al-Baghdadi's death, according to a new report from Joby Warrick, Ellen Nakashima, and Dan Lamothe of The Washington Post.

The mole, whose nationality was not disclosed, was so deep in al-Baghdadi's inner circle that he was trusted to escort members of the ISIS leader's family to medical appointments, officials familiar with the matter told The Post.

The outlet said the informant was present during the raid over the weekend by US Delta Force operators.

The informant and his family were removed from the region two days after the raid, and he may receive some or all of the $25 million bounty the US had put on al-Baghdadi, The Post said.

The informant is believed to be a disillusioned ISIS member who was initially cultivated by the Kurdish-led Syrian Democratic Forces. The Post cited one official as saying the mole turned after a family member was killed by ISIS.

SDF leaders then brought in US intelligence officials, who went on to vet the source, US and Middle Eastern officials familiar with the operation told The Post.

The SDF's commander, Gen. Mazloum Abdi, has said publicly that an SDF asset was key to the operation against al-Baghdadi, telling NBC News the source had taken items from al-Baghdadi's compound, including used underwear about three months ago, so that US operatives could confirm that al-Baghdadi was indeed in the compound.

The source's knowledge of al-Baghdadi's hideout — he apparently oversaw construction of the safe house — proved particularly vital to the operation, The Post said, as was intelligence that al-Baghdadi always had an explosive vest on him to avoid capture.

"It was assessed for quite a while that the person might have the key to the lock," one of the US officials told The Post, which reported that the informant began assisting in the operation over the summer. "That only really seriously became clear within the last couple of weeks."

Al-Baghdadi had been notoriously difficult to track, and Saturday's raid was the culmination of years of work by the US and partner forces, which have been embedded in the region on the hunt for al-Baghdadi since 2015. He eschewed mobile phones and trackable devices and frequently moved location.

In fact, The Post said, the informant was entrusted with helping al-Baghdadi change locations around Syria's Idlib province, with the leader and his entourage ultimately ending up in the compound in the village of Barisha where the raid took place.

The informant turned US forces to Idlib — an unlikely place for the Iraqi al-Baghdadi to hide out, especially given that it is under Russian air control and militia factions control the ground — and US and French forces zeroed in on al-Baghdadi over the summer.

The Pentagon and the White House haven't commented on the presence of an informant in al-Baghdadi's inner circle.

"I'm not going to comment on what may or may not have happened with the SDF on the objective," Gen. Mark Milley, the chairman of the Joint Chiefs of Staff, told reporters Monday when asked whether the SDF was involved with Saturday's operation.

"The actions on the objective, the aircraft coming in, the aircraft overhead, and the soldiers conducting the assault, was a US-only operation," he said.