A top ISIS official's death could have major implications for the future of the group

The death of the ISIS leader who oversaw external attacks could have significant implications for the group as it pivots from seizing territory in the Middle East to launching attacks on Western targets.

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The terrorist group announced on Tuesday that its spokesman, Abu Muhammad al-Adnani, had been killed near Aleppo, Syria. His apparent loss marks a major blow to a group that's already struggling for long-term survival.

"This really sends the message out that ISIS is truly on the decline because he was such a figurehead," Clint Watts, a fellow at the Foreign Policy Research Institute and former US Army officer and FBI Joint Terrorism Task Force special agent, told Business Insider. "[A]dnani was such a key person for building support, propaganda, and online recruitment. He was a key figure."

Adnani's death could hurt the group's attack capability in the long term, Watts said. And it's unclear whether ISIS has a successor in line to take over Adnani's role in the group.

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Colin P. Clarke, a terrorism expert and political scientist at the RAND Corp., speculated that it won't be easy finding a replacement for Adnani.

"Replacing Adnani will be hard," Clarke tweeted. "He had logistical/comms expertise which isn't easily replicated but learned through tacit knowledge transfer."

Clarke called Adnani's death "a severe blow to the group's external-operations network."

Bridget Moreng, an analyst who studies ISIS's global strategy, noted that in addition to leading external operations, Adnani was thought to be the next in line to lead ISIS in the event of "caliph" Abu Bakr al-Baghdadi's death.

And because it's not clear who might be taking over Adnani's job, there could be some infighting that would further fracture the group.

"Key question is how IS will adjust to Adnani's death: smooth succession or power vacuum/infighting for position," Moreng tweeted. "The latter is likely."

But terrorist groups like ISIS are generally equipped to survive the deaths of top leaders. Al Qaeda, for instance, is still a major player in the world of terrorism despite the death of its founder, Osama bin Laden.

"These leadership strikes, they're important," Thomas Joscelyn, a senior fellow at the Foundation for Defense of Democracies and an expert on terrorist groups, told Business Insider. "Killing top leaders definitely weakens the organization. But a lot of the time, they have people to replace them that nobody knew about."

Al Qaeda "has replaced its external operations chief over and over again," Joscelyn said.

"The loss of leadership definitely weakens organizations, and it may lead to a further degradation of ISIS's capabilities, but we've seen them adapt and survive in the past," he said. "These organizations aren't built around one or two guys. They have a deep bench of leadership."

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ISIS also has plenty of lower-ranking operatives responsible for coordinating external attacks.

"A lot of times the middle managers are the ones who make things happen," Joscelyn said. They "have people who have detailed local knowledge who they rely on for tactical planning in operations in the West."

Still, ISIS has recruited its base of support around its message of "remaining and expanding," not simply surviving. And Adnani's death could hurt morale within the group as it continues to lose territory in Iraq and Syria. ISIS might try to execute more plots against the West to head off any drop in morale or perception that the group is being defeated.

"What you may see is an actual increase in attacks," Watts said. "Whoever [Adnani's] successor is may want to be more aggressive. If there's any in the pipeline, they might be accelerated by Adnani's death."

Past the short term, however, Adnani's death might not inspire many future attacks. Unlike Anwar al-Awlaki, the notorious preacher and Al Qaeda recruiter, Adnani didn't speak English or build up a library of speeches online that supporters — or curious individuals — could access.

"When you think about it, who will be talking about Adnani in two years?" Watts wondered. "I don't know."