Niger's military has detained a suspect who could be the militant leader who was being pursued at the time of an ambush that left four American soldiers dead.US Ambassador Eric P Whitaker told The Associated Press on Tuesday that he does not know the identity of the detained suspect, but that the head of Niger's special forces is hopeful it's a known extremist leader.At the time of the deadly October ambush, US forces and their counterparts from the Niger military were pursuing Doundou Chefou, a militant suspected of involvement in the kidnapping of an American aid worker.Whitaker says security forces in Niger have been detaining suspected extremists in the country's western Tillabéri region. Authorities on Tuesday are awaiting identification of whether the man in custody is Chefou."Detentions by Nigerian forces are ongoing," Whitaker said.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook A US investigation into the October ambush, which was claimed by fighters linked to the Islamic State group, has not yet been released."Regrettably, they were ambushed by ISIS Greater Sahara forces," said Whitaker.US officials familiar with the military investigation into the Niger ambush said last month that it concluded the team didn't get required senior command approval for their risky mission to capture Chefou. As a result, commanders couldn't accurately assess the mission's risk, according to the officials who spoke on condition of anonymity to discuss the results of the not-yet-released investigation.The investigation finds no single point of failure leading to the attack, which occurred after the soldiers learned Chefou had left the area. It also draws no conclusion about whether villagers in Tongo Tongo, where the US team stopped for water and supplies, alerted ISIS militants to American forces in the area. Still, questions remain about whether higher-level commanders — if given the chance — would have approved the mission, or provided additional resources for it.Before October, there had not been any major incursion like that into Niger before, said the commander of Special Operations Command Africa, Maj. Gen. Marcus Hicks, who warned that the extremist threat has been marching south in the Sahel at an unprecedented level.Threats in the region include al-Qaida-linked fighters in Mali and Burkina Faso, Islamic State group-affiliated fighters in Niger, Mali and Nigeria and the Nigeria-based Boko Haram. All take advantage of the vast region's widespread poverty and poorly equipped security forces.US special operations forces have been advising local troops on the continent for years, just under 1 000 across Africa.Even before the October attack, the US began to shift away from assisting tactical units on the front lines toward training, advising and assisting farther up the chain of command at the battalion level, Hicks said.