The Kurdish-led Syrian Democratic Forces (SDF) say their spy stole Abu Bakr al-Baghdadi's underwear which was then DNA tested and used to prove his identity before he was killed.
Senior SDF commander Polat Can claims their source also played a vital role in tracking down the Islamic State (IS) leader's location before an operation by US special forces in Syria.
Baghdadi killed himself in the raid.
US President Donald Trump has downplayed the role of Kurdish forces.
When announcing the raid on 27 October, Mr Trump said the Kurds provided "helpful" information but added that they had not carried out "a military role at all".
Mr Can insisted the SDF played an important part in the raid in a Twitter thread posted on Monday.
"All intelligence and access to al-Baghdadi as well as the identification of his place were the result of our own work. Our intelligence source was involved in sending coordinates, directing the airdrop, participating in and making the operation a success until the last minute," he said.
3 – Our own source, who had been able to reach Al Baghdadi, brought Al Baghdadis underwear to conduct a DNA test and make sure (100%) that the person in question was Al Baghdadi himself.
— بولات جان Polat Can (@PolatCanRojava) October 28, 2019
End of Twitter post by @PolatCanRojava
Mr Can added that the SDF had been working with the CIA to track Baghdadi since 15 May, and had discovered that he was hiding in Idlib province, where the raid took place.
The source, says Mr Can, had found out that the IS leader was about to move to a new location in Jarablus.
The SDF have been key allies of the US in the battle against the Islamic State (IS) group, but earlier this month Mr Trump pulled American troops out of northern Syria.
Analysts say the US withdrawal in effect gave Turkey the green light to begin a cross-border assault on the region.
What do we know about the raid?
Several US allies or powers in the region were given advance notification of the raid, including Turkey, Iraq, Kurdish forces in north-eastern Syria, and Russia, which controls airspace over Idlib.
The troops arrived to a barrage of shots from the ground, reports said.
On landing, the US force called on Baghdadi, who had fled into a tunnel, to come out and surrender.
The force blew holes in the walls to avoid any booby traps in doors. The retreating Baghdadi then detonated his suicide vest, killing himself and three children in the tunnel.
Mr Trump said test results carried out on the remains "gave certain, immediate and totally positive identification" that it was Baghdadi.
The tests were carried out on site by technicians who accompanied the special forces perRead More – Source