The Islamic State militant group confirmed on Thursday its leader Abu Bakr al-Baghdadi has been killed, the group's news agency Amaq said in an audio tape following a U.S. weekend raid.

The extremist group also confirmed its spokesman Abu al-Hassan al-Muhajir was also dead. Al-Muhajir, who The New York Times reported would be Baghdadi's most likely successor, was first reported killed on Sunday.

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The group had been silent until now and as successor it appointed someone Amaq only identified as Abu Ibrahim al-Hashemi al-Quraishi.

An ISIS spokesman addressed the United States in the tape.

“Beware vengeance (against) their nation and their brethren of infidels and apostates, and carrying out the will of the commander of the faithful in his last audio message, and getting closer to God with the blood of polytheists,” he said.

Aymenn al-Tamimi, a researcher at Swansea University focused on Islamic State, said the name was unknown but could be a top figure called Hajj Abdullah Qardash whom the U.S. State Department had identified as a possible successor to Baghdadi

"It could be someone we know, who perhaps has just assumed this new name," said Tamimi.

The group, which controlled swathes of Iraq and Syria between 2014 and 2017 and carried out atrocities that horrified most Muslims, also confirmed the death of its spokesman Abu al-Hassan al-Muhajir.

The National previously reported, Abdullah Qardash, an Iraqi of Turkmen origin, had been named by Baghdadi as his successor in August.

Experts and security officials had predicted one of al-Baghdadi’s close aides would likely succeed him. But the shadowy head of ISIS was security-obsessed and known for turning on members of his close circle. One possible favorite was Abdullah Qardash - the two met in a U.S. prison in 2003 and Qardash became al-Baghdadi’s top security henchman.

The ISIS Shura council, the shadowy group’s leadership of 10 or so people, is supposed to choose the next leader if al-Baghdadi had designated a successor. But it’s not clear how many of them are still alive.

The Associated Press contributed to this report