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A group in northwestern Africa that is loyal to the Islamic State of Iraq and Syria issued a statement claiming responsibility for the October attack in Niger that killed four U.S. soldiers who were on patrol with Nigerien forces.

The statement offered no explanation for the delay in claiming responsibility for the Oct. 4 attack, which U.S. officials had said was probably carried out by the group.

“We declare our responsibility for the attack on the U.S. commandos last October in the Tongo Tongo region of Niger,” said the statement, attributed to Adnan Abu Walid al-Sahraoui, who was a member of Al-Qaida’s regional branch before pledging allegiance to ISIS two years ago.

The statement was issued to reporters at a website in Mauritania to which fighters from Sahraoui’s group have previously sent missives.

The assault last fall was one of the most deadly recent attacks on U.S. soldiers in Africa. In addition to the four Americans, including two members of the Green Berets, five Nigerien soldiers who were with them on a joint mission were killed.

Details of the attack remain murky, and members of the patrol have given conflicting accounts of it. It is unclear whether the patrol was simply ambushed, or whether it was attacked after the troops were reassigned to support a separate, clandestine counterterrorism mission against Islamic militants in the area.

Aid workers and tourists have long been urged to avoid the area where the attack occurred, near Niger’s border with Mali, because of the presence of both Al-Qaida- and ISIS-affiliated groups.

The extent of Sahraoui’s ties with ISIS is unclear. The website in Mauritania that carried the group’s statement Friday is an outlet favored by Sahraoui’s former colleagues in Al-Qaida, not by ISIS. The area in which Sahraoui’s group operates contains some of the most forbidding terrain on the planet, a landscape of undulating dunes where cellphone towers are few and far between.

“There is a lot we don’t know about how his operation connects back to the mother ship — what’s the connective tissue?” said Thomas Joscelyn, an analyst who has tracked the group for years as a senior fellow at the Foundation for Defense of Democracies. “There are a lot of possibilities and many factors in play.”

The remoteness of the area in which Sahraoui’s group operates, and the difficulty of getting reliable cellphone signals or internet access, could be one factor to explain the delay in releasing the statement. Another possibility is that ISIS’ media apparatus was disrupted after the group lost nearly 98 percent of its territory in Iraq and Syria.

Additionally, there have been reports of unrest among from Al-Qaida loyalists after Sahraoui made his pledge of allegiance to ISIS. “There were even reports at one point that he was injured in a shootout with Al-Qaida,” Joscelyn said.

Sahraoui cut his teeth in Al-Qaida’s branch in the region, which reported to Osama bin Laden through letters that were carried across the desert by couriers. He joined the Qaida branch sometime in 2010, according to one account, and became a deputy to Abdelhamid Abu Zeid, one of Al-Qaida’s most notorious commanders in the area and among the first to discover that foreigners were lucrative bargaining chips.

He bankrolled his operations through kidnappings for ransom, pioneering a business model that was later adopted by the terrorist group in Yemen, Syria, Afghanistan and elsewhere in the region.