The Islamic militants came on motorcycles toting rocket-propelled grenades and heavy machine guns and killed 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. The terrorists were helped by operational confusion - Niger blocked a U.S. assistance aircraft and French air support, in which the level of coordation between the U.S. and France remains unclear.

Who were the attackers and how did they know the Americans would be there?

Open gallery view Map locates Niamey, Niger, where 3 US commandos were killed; 1c x 2 1/2 inches; with BC-US--United States-Niger; JEM; ETA 7 a.m. SOURCE: maps4news/HERE, U.S. Africa Command Credit: AP

No extremist group has claimed responsibility for the deadly ambush on October 4 and the languages reportedly spoken by the jihadists are used throughout the Sahel.

The ambush of U.S. troops in Niger has become the center of controversy in America as President Donald Trump is criticized, including by a grieving family, for the way he spoke to the wife of one soldiers slain in the operation. Some U.S. lawmakers are also calling for a Benghazi-like investigation.

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

Who evacuated American forces?

For several years, American and French forces have provided training and support to the militaries of Mali, Niger and other vulnerable African countries where Islamic extremism has become increasingly entrenched over the past decade. CNN reported that one privately contracted aircraft and French military helicopters helped evacuate American forces and the three bodies, but details remain unclear if "both parties shared all necessary information related to the operation."

The body of Sgt. La David Johnson, the fallen Green Beret at the center of the controversy surrounding Trump's comments, was originally left behind in Niger during the confusion and returned to the U.S. via Miami on Tuesday.

American hostage

Some officials believe Walid’s militants are also holding hostage Jeffery Woodke, an American 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 militant 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 jihadist occupation in 2012.

Al-Qaida affiliation

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

In the year since then he has called for attacks on foreign tourists in Morocco and the UN mission in Western Sahara, according to audio messages released in his name. It is not clear if Walid is receiving financial help from the Islamic State group 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 close to 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 Islamic State in the Sahel does not yet pose a threat as great as the al-Qaida 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 already facing an escalation in violence across the Sahel. A report by the UN chief obtained this week warned that the security situation in the Sahel is in “a continuous downward spiral.”

Now the UN is urging the international community to finance a 5,000-strong regional force, with the head of the UN saying “the stability of the entire region, and beyond, is in jeopardy.”

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

And the future of the regional security force known as the G5 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 UN 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 operationing 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 G5 would come out stronger.

Jihadist groups, though, are infiltrating the population and 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.”