NIAMEY, Niger -- On Tuesday, the investigation into the attack in Niger that killed four U.S. soldiers was focusing there on hunting down those responsible.

American soldiers had been on a patrol with Nigerian forces seeking information on a high-value terrorist target on October 4, when they were ambushed. The troops, not expecting enemy contact, came under fire in what was described by U.S. military sources as a "complex attack," involving small arms fire and rocket-propelled grenades.

One Nigerien soldier, who was stationed at the base nearest to the site of the ambush, told CBS News correspondent Debora Patta that he knew the four Americans who were killed personally, and that they had trained him in counter-terrorism tactics. He said he recalled cracking military jokes with them despite the language barrier, Patta reports.

The soldier said he believed the ambushed troops were deliberately delayed in the village of Tonga Tonga, in a region known as Tillaberi, where the Nigerien government has no control. Nigerien authorities were investigating the suspicion that some of the villagers may have been complicit in the attack.

Niger is one of the most remote and chaotic war zones in the world, Patta reports. There are over a dozen extremist groups operating along its border with Mali, and this particular attack was coordinated by ISIS in the Greater Sahel - a group led by Abu Walid Al Zaharwoui - which has been actively recruiting in the region.

The presence of American troops in the area would have drawn immediate attention, Patta reports, which begs the question why they went into such a dangerous location with very little back up.