“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 in Washington. “There are a lot of possibilities and many factors in play.”

The remoteness of the area in which Mr. 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 the Islamic State’s 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 Qaeda loyalists after Mr. Sahraoui made his pledge of allegiance to the Islamic State. “There were even reports at one point that he was injured in a shootout with Al Qaeda,” Mr. Joscelyn said.

Mr. Sahraoui cut his teeth in Al Qaeda’s branch in the region, which reported to Osama bin Laden through letters that were carried across the desert by couriers. He joined the Qaeda branch sometime in 2010, according to one account, and became a deputy to Abdelhamid Abu Zeid, one of Al Qaeda’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.

By 2011, Mr. Sahraoui was in charge of taking care of foreign hostages kidnapped by the group, according to Mariasandra Mariani, an Italian who was held by him for more than a year after her abduction in Algeria on Feb. 2, 2011.

He parted ways with Al Qaeda in 2012, after the jihadists seized most of northern Mali. He resurfaced as the spokesman of the Movement for Oneness and Jihad in West Africa, a separate jihadist group based in Mali, which merged with a third group in 2013.

Then in May 2015, he swore loyalty to the Islamic State leader, Abu Bakr al-Baghdadi. But his video pledge was not released by the group’s news agency for more than a year, until October 2016. Since then, Mr. Sahraoui’s statements have not been promoted on ISIS media outlets, including Friday’s claim, which was sent to the Mauritanian website. The Islamic State affiliate operating in Nigeria regularly succeeds in uploading messages and photo essays through established ISIS media channels.