BAMAKO, Mali: The Islamic militants came on motorcycles toting rocket-propelled grenades and heavy machine guns, killing four American service members after shattering the windows of the unarmored U.S. trucks. In this remote corner of Niger where the Americans and their local counterparts had been meeting with community leaders, residents say the men who came to kill that day had never been seen there before.

“The attackers spoke Arabic and Tamashek, and were light-skinned,” Baringay Aghali, told the Associated Press by phone from the remote village of Tongo-Tongo.

Who were these men and how did they know the Americans would be there that day?

No extremist group has claimed responsibility for the deadly ambush on Oct. 4 and the languages reportedly spoken by the militants are used throughout the Sahel including Tamashek, spoken by ethnic Tuaregs.

The ambush of U.S. troops in Niger has been the center of controversy in America because President Donald Trump has been criticized in some quarters, including by one grieving family directly, for the way he spoke to the wife of one of the soldiers slain in that operation.

The Niger attack appears to be the work of the Daesh (ISIS) of the Sahel, a splinter group of extremists loyal to Daesh who are based just across the border in Mali, according to interviews with U.S. officials and authorities here in the vast Sahel region bordering the Sahara Desert. It is led by Adnan Abu Walid who built ties with various extremists before forming his own group.

Some officials believe Walid’s militants are also holding an American, Jeffery Woodke, who was abducted in Niger a year ago. A rebel leader approached by Niger authorities to conduct negotiations for his release confirmed that Walid’s group is holding Woodke, who had spent 25 years as an aid worker in Niger, one of the poorest countries in the world.

Now Walid’s group is suspected of the attack that killed four American soldiers this month.

The ambush in Niger highlights how extremist groups have shifted and rebranded since the 2013 French-led military operation ousted them from power in northern Mali. Those extremists lost Mali’s northern cities but regrouped in the desert, including the man suspected of ordering the attack on the Americans.

Walid, 38, also known in some circles as Adnan al-Sahrawi, descends from the Sahrawi people, who are found across southern Morocco, Mauritania and parts of Algeria. He has long been active with Islamic extremists in Mali, at one time serving as the spokesman of the Mali-based group known as MUJAO that controlled the major northern town of Gao during the militant occupation in 2012.

That group was loyal to the regional Al-Qaeda affiliate. But Walid parted ways and in October 2016 a video circulated on the internet in which he pledged allegiance to Daesh in Iraq and Syria.

In the year since then he has called for attacks on foreign tourists in Morocco and the U.N. mission in Western Sahara, according to audio messages released in his name. It is not clear if Walid is receiving financial help from Daesh or if the links are purely ideological.

Walid’s following now includes numerous members of the Peul ethnic group in the Mali-Niger border areas, who are active in the area near where the attack on the U.S. soldiers took place. Before the attack on the U.S. troops in Niger, Walid’s followers are believed to have staged a series of bloody attacks on military installations in Niger. In February, they were blamed for an assault in Tliwa where a dozen Niger soldiers were slain.

Walid’s Daesh in the Sahel does not yet pose a threat as great as Al-Qaeda militants in the region though that could shift with time, said Ibrahim Maiga, with the Institute for Security Studies in Bamako. Walid clearly appears to have learned from his former colleagues on how to infiltrate and influence locals, he said.

“He has succeeded ... in creating links with local people despite the fact that he is a stranger to the area,” he said.

The growing threat posed by Walid’s group comes as the international community is facing an escalation in violence across the Sahel. A report by the U.N chief obtained this week by the AP warned that the security situation in the Sahel is in “a continuous downward spiral.”

For several years American and French forces have provided training and support to the militaries of Mali, Niger and other vulnerable countries in this corner of Africa where Islamic extremism has become increasingly entrenched over the past decade. Now the U.N. is urging the international community to finance a 5,000-strong regional force, with the head of the U.N. saying “the stability of the entire region, and beyond, is in jeopardy.”

The 12,000-strong U.N. peacekeeping mission in Mali has become the most dangerous in the world as Islamic militants routinely attack U.N. convoys across the north.

And the future of the regional security force known as the G-5 Sahel Multinational Force – made up of Burkina Faso, Chad, Mali, Mauritania and Niger – appears to be in jeopardy.

France, the former colonizer which has a 5,000-strong military operation to help stabilize the region, has been a major financial backer. Funding, though, has come up short.

The Security Council unanimously adopted a resolution in June welcoming the deployment, but at U.S. insistence it did not include any possibility of U.N. financing for the force. So far only one-quarter of the needed funds have been raised, throwing into doubt whether the regional forces will begin operations this month as scheduled.

Maiga, the Malian security expert, said winning the battle against extremism will not be only a question of firepower. If it were a conventional conflict with two armies respecting roughly the same rules, the G-5 would come out stronger.

Militant groups, though, are infiltrating the population, exploiting the absence of government in some of these remote areas. That is how Walid’s group may have learned about the visit of the U.S. troops to local communities. Within the communities where troops are attacked, someone is tipping off the extremists.

“The outcome of this battle will not depend solely on the size of the troops,” he said, “but also on the ability of states to regain the confidence of the population.”