ISIS expert: Some of the Paris attackers were likely trained in ISIS territory

Investigators still don't know the details of how terrorists were able to carry out separate, simultaneous attacks in Paris on Friday night.

But the complexity of the operation suggests that some of the assailants might have had training in the Middle East, experts say.

A group of seven terrorists took hostages, detonated suicide vests, and shot people in attacks across Paris on Friday night, killing 129 and wounding more than 300. ISIS (also known as the Islamic State) has claimed responsibility for the attacks, but it's so far unclear where specifically the attackers came from - or how exactly they organized the massacre.

But the careful coordination required to carry out simultaneous attacks in a major city with a well-developed anti-terrorism system suggests that at least some of the attackers have experience and training, according to Will McCants, an expert on jihadism and author of the recent book, "The ISIS Apocalypse."

"Anything is possible, but the complexity of the attacks, the armaments that they had, suggests that it's not merely inspiration or activating a local cell," McCants told Business Insider on Saturday. "It suggests that at least some of the people had training in a theater of war or in ISIS-controlled territory" in Syria and Iraq.

Daveed Gartenstein-Ross, a counterterrorism analyst and senior fellow at the Foundation for Defense of Democracies, agreed with this assessment.

"If I were placing a bet, that's where I would place my bet," Gartenstein-Ross told Business Insider. "That would be my initial hypothesis that they had received some training. I think it's likely, but I don't think it's certain."

The Paris attacks are stunning in their scope and effectiveness. Attackers were able to simultaneously detonate a suicide vest outside of a stadium during a soccer game, open fire on a restaurant and cafe in the city center, and take hundreds of hostages at a club while a rock band was playing a concert.

And they were able to execute these attacks under the nose of French authorities who have been on high alert since Al Qaeda operatives attacked the satirical magazine Charlie Hebdo earlier this year.

But despite early indications pointing to formal training, it's still too soon to tell for sure.

ISIS's claim of responsibility was lengthy compared to the other statements the group regularly releases through its propaganda channels online, but the statement was short on details aside from what had already been reported. This makes it hard to know how the attack was planned and carried out and how involved ISIS's core leadership was in the coordination.

"I still think it's an open question," Gartenstein-Ross said. "It could be that … [ISIS] didn't order it and that some of their fighters carried it out without them having knowledge. It could have been that they gave a general order."

Investigations into the attacks are ongoing.

At least one of the attackers was reportedly known to French authorities. Paris prosecutor Francois Molins said on Saturday that one of the terrorists was a 29-year-old French national who had a criminal history and had been radicalized in 2010, according to CNN.

Officials have also said that one of the bombers is thought to have entered Europe as a Syrian migrant. French authorities reportedly matched the remains of one of the suicide bombers to a Syrian passport that was used to apply for asylum in Europe.