Pentagon officials said Thursday they believe a self-radicalized, Islamic State-affiliated group was responsible for the deadly attack in Niger last week that left four Green Berets dead.

Joint Staff Director Lt. Gen. Kenneth McKenzie told reporters that the patrol of U.S. and Nigerien forces attacked last week had “done 29 patrols without contact over the previous six months or so,” with no indication that the Oct. 4 ambush was to occur.

“ISIS still have a powerful message in the cyber world, self-radicalization occurs around the world, that is an enduring problem,” McKenzie said. “So if I had to guess, and I’m guessing, I would say these are probably self-radicalized. But there is also some minimal flow of people across the divide.”

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The group of a dozen U.S. soldiers were ambushed while conducting a joint patrol with about 40 Nigeriens. Four Green Berets were killed and two were injured.

“Clearly I think we believe probably some form of ISIS affiliation, the group that we’re talking about,” McKenzie said. “We’re still looking into specific details with them.”

The American and Nigerien forces were able to call in French air support 30 minutes into the attack, which did not drop any munitions on the enemy attackers, but was “effective, we believe, in bringing the engagement to an end,” McKenzie said.

The Defense Department works closely with the French military, which has about 5,000 forces in and around Mali, west of Niger.

The United States has about 1,000 troops in the Chad River Basin, which is spread out across Niger, Chad and the top of Nigeria and the Central African Republic. Most of the troops are in Niger, McKenzie noted.

McKenzie said he believes the attack “was a natural product of the fact that [ISIS is] being crushed in the core caliphate,” by the U.S.-led coalition in Iraq and Syria.

“[They’re] trying to go to other places, trying to find the cold corner of the world. While this particular instance is tragic, it’s also illustrative of the general success of the campaign,” he said.

He emphasized, however, that he believes the attack was carried out not by ISIS militants moving from the Middle East to Africa, but by a self-radicalized regional group.

“I’d be very surprised if they were foreign fighters that came from Syria,” he said.

No group has yet claimed responsibility for the attack, which took place along the border between Niger and Mali.

Multiple extremist groups operate in the region including, in Mali, al Qaeda in the Islamic Maghreb and Boko Haram, an ISIS-aligned group based in Nigeria.