Russia Claims Airstrike May Have Killed ISIS Leader Abu Bakr al-Baghdadi

Russia’s defense ministry announced on Friday that it is investigating whether it killed ISIS leader Abu Bakr al-Baghdadi in an airstrike in Syria on May 28.

Baghdadi’s killing or capture would be a significant symbolic blow to the weakened Islamic State. Baghdadi is not only the group’s top leader, he also declared himself the emir al-muminin, the leader of the faithful, claiming the mantle of the early Islamic empires dating back to the seventh century in defiance of rival jihadist groups, such as al-Qaeda.

The jihadi leader’s June 2014 sermon in Mosul was one of his last confirmed public appearances. The mosque of al-Nouri, where he spoke, is now besieged by Iraqi troops battling to recapture the last remaining pocket of ISIS territory in Mosul.

The Russian claim does raise the question of Baghdadi’s whereabouts as the Islamic State leadership reels from a massive assault on its strongholds in Iraq and Syria. The group now holds only a small besieged section of Mosul, the city that had been the largest under its controlled. Separately, U.S.-backed militias are advancing on Raqqa, the Islamic State’s putative capital. The Islamic Caliphate that Baghdadi himself declared in a sermon in Mosul in June 2014 is now collapsing, leaving a shrinking space for its leaders to hide. Asked about Russia’s assertion, a spokesperson in for the U.S.-led military coalition battling ISIS told TIME, “We cannot confirm these reports at this time.”

Yet, much like the rumors of Mark Twain’s demise in 1897, past reports of the jihadist leader’s death have been exaggerated. Numerous claims that Baghdadi has been injured or killed have surfaced over the last three years, with no definitive proof of his fate. The last such claim came on June 10 when Syrian state television reported that he had killed in a regime airstrike on the ISIS-held city of Raqqa.

His whereabouts have long been a subject of much speculation, based on little concrete evidence. In 2015 The Guardian reported Baghdadi had at one point been hiding out in the town of Ba’aj, in northern Iraq, where he survived an airstrike that injured his spine. Iraqi forces have now retaken the town, and commanders there told the newspaper that ISIS leaders had fled over the border to Syria.

Baghdadi’s voice resurfaced in November 2016 as U.S.-backed Iraqi forces began their offensive on the city of Mosul. In the recording attributed to Baghdadi, he urged his followers to fight to the death, calling the battle a “total war.” Speaking in response to the recording, British Foreign Secretary Boris Johnson said that the jihadi leader had in left Mosul. American officials pointed to the tape as evidence that Baghdadi had been driven far underground.

“He is in deep, deep hiding at best, able from time to time to issue an audio tape which is not the most inspiring form of leadership in the digital media age,” Brett McGurk, the U.S. presidential envoy to the anti-ISIS coalition, said in Germany in November 2016.

Counterterrorism experts say Baghdadi and other senior ISIS leaders have adopted strict measures to avoid detection by hostile intelligence services. “The guy wouldn’t allow cell-phones anywhere near him. He eschewed suggestions or attempts to provide him with a body-guard detail because he didn’t want to be noticeable. He reportedly wouldn’t allow the internet in his places of residences,” says terrorism analyst Michael S. Smith in a recent interview with TIME.