Washington-The reported death of ISIS’ senior propagandist and strategist, Abu Mohammed al-Adnani, in an American drone strike in northern Syria on Tuesday casts in sharp relief the immediate challenge the terrorist group faces in replacing one of its pivotal founding members.

The attack, carried out by a military Reaper drone, also underscores the progress the military’s most elite Special Operations commandos and the Central Intelligence Agency have made in the conflict’s two years by using information from spies on the ground and sensors in the sky to target a growing number of ISIS leaders.

The American-led coalition has killed about 120 important ISIS officials and operators, including about a dozen of the group’s top leaders, according to the Pentagon.

Still, the ISIS has proved to be remarkably resilient, American officials and counterterrorism specialists say, noting that the group has succession plans to replace even its top leader, Abu Bakr al-Baghdadi, should he be killed.

“There’s a deep bench,” said William McCants of the Brookings Institution, the author of “The ISIS Apocalypse.”

In the coming days, Baghdadi is likely to meet with his shura, or council of advisers, in Raqqa, the group’s self-proclaimed capital in Syria, to pick a replacement for Adnani, a 39-year-old Syrian, who had been believed to be Baghdadi’s heir apparent.

Among the candidates to replace Adnani is Turki al-Binali, 31, one of the most senior clerics of ISIS, who is believed to have been appointed the group’s chief mufti.

A native of Bahrain, Binali is considered a prodigy who studied under some of the top leaders in the jihadi pantheon, including Abu Muhammad al-Maqdisi, who is regarded as one of the most influential Al-Qaeda ideologues.

His nationality was later withdrawn after he was accused of terrorism.

“Even more important than that is that he is an extremely talented speaker, orator — kind of like Adnani,” said Cole Bunzel, a doctoral candidate at Princeton who who wrote a Brookings Institution paper on the ideology of the ISIS.

As early as 2013, Binali is believed to have traveled to join ISIS in Syria, where he began producing some of the group’s most influential theological treatises which laid the foundation for the group’s future actions.

On April 30, 2014, he published an essay arguing that one does not need to have full territorial control before declaring an Islamic caliphate.

The concept of territorial control was believed by many to be a prerequisite for a caliphate, said Bunzel, and Binali’s essay served to clear the way for the declaration of the caliphate months later in the summer of 2014.

Bunzel said he suspected that it was Binali who headed ISIS’ Research and Fatwa Department, which issued pamphlets explaining the legality of raping enslaved Yazidi women.

United States intelligence officials said Binali may have also traveled recently to Libya to help ISIS bolster its franchise there, an affiliate group that in recent weeks has been driven out of its stronghold in Surt.

Laith Alkhouri, a director at Flashpoint, a business risk intelligence company in New York that tracks militant and cyber threats, said that while Binali is certainly a contender for a greater leadership role, he may have been marginalized in recent months.

If so, Alkhouri said, another candidate could be Abu Luqman, also known as Ali Mousa Al-Shawwakh, who was the first wali, or ruler, in Raqqa and led much of the ISIS strategy in Aleppo in 2015.

Alkhouri said Luqman fits the characteristics of someone ISIS would trust as spokesman. He was imprisoned by the Syrian government before the revolution, holds a law degree and worked as a recruiter for the forerunner of ISIS during the United States occupation of Iraq.

Luqman has been reported killed several times: once in an airstrike, another time by being stabbed by a Libyan fighter who was displeased by Luqman’s treatment of foreign fighters. His death has never been officially confirmed.

The person who takes Adnani’s place will undoubtedly work closely with Baghdadi, the organization’s shadowy leader.

Baghdadi meets periodically with regional emirs at his headquarters in Raqqa.

To ensure his safety, special drivers pick up each of the emirs and take their cellphones and any other electronics to avoid inadvertently disclosing their location through tracking by American intelligence, United States officials said.

The high stakes at play in the apparent death of Adnani were revealed on Wednesday when the Russian military said that its warplanes had carried out a strike in Syria that killed up to 40 ISIS fighters, including Adnani.

The Russian announcement appeared to be a direct challenge to the Pentagon, which claimed a day earlier that one of its drone strikes had killed the senior figure.

Pentagon officials dismissed Russia’s claim, but have not confirmed the reports by ISIS’ official news agency that Adnani was killed.

Russia has faced criticism from the West that its military intervention in Syria was undertaken to prop up Bashar Assad of Syria, rather than to fulfill the Kremlin’s stated goal of fighting terrorism.

This is the first public announcement from Russia that it had killed a specific senior figure in the terrorist group.

For more than two years, Adnani had a $5 million bounty on his head, offered by the United States, and he was on the kill list for the American-led air campaign in Iraq and Syria.

It is unclear how the United States identified and tracked Adnani, but his death, if confirmed, would highlight the ability of American agencies to collect and coordinate information gathered from raids on ISIS safe houses.

Such raids produce intelligence from cellphones and computer hard drives and other information that is combined with an increasingly effective network of spies and informants to put pressure on ISIS leaders.

That pressure may have been why Adnani was in the region of northern Syria where his vehicle was struck by Hellfire missiles on Tuesday night.

Al Bab, where Adnani’s vehicle was hit, has become one of several hubs for ISIS operatives in recent years, in part because it is the largest population center near the Turkish border that the group still controls, a place where ISIS figures can try to disappear among what’s left of an urban population.

The New York Times