 -- U.S. military investigators visited a remote area of northern Niger on Sunday that was the scene of a deadly Oct. 4 ISIS ambush that killed four American soldiers and four Nigerien soldiers.

The U.S. military's investigation into the circumstances of the deadly ambush is expected to conclude in January.

"A joint U.S. Africa Command and Niger military investigation team returned to the village of Tongo Tongo, Niger on Nov. 12 in order to gain a clearer understanding of the Oct. 4 ambush, the attack site and the surrounding environment," said a statement from U.S. Africa Command (AFRICOM).

"This mission allowed the investigation team to gather information and determine the facts related to the ambush that killed four U.S. soldiers and four Niger Force Soldiers," it added.

On Oct. 4 a dozen U.S. Army advisers and the 30 Nigerien soldiers they were accompanying came under attack shortly after leaving Tongo Tongo.

Staff Sgt. Bryan Black, Staff Sgt. Jeremiah Johnson, Staff Sgt. Dustin Wright and Sgt. La David Johnson were killed in the attack carried out by ISIS-Greater Sahara, a small ISIS affiliate.

According to Africa Command the investigation team interviewed local villagers, carried out a physical examination of multiple areas of interest related to the attack "and retraced actions leading up to, during and after this ambush."

Located close to the border with Mali, Tongo Tongo is located in an area that was already known as a hotbed of Islamic extremist groups even prior to the deadly ambush.

AFRICOM said it "went to great lengths" to ensure the safety of investigators and to protect the operational security of the visit.

"Sufficient assets were available in the event we had to respond to an attack that threatened innocent civilians, our partner forces or our own personnel," said the AFRICOM statement.

Media outlets that have visited Tongo Tongo since the attack have done so with heavy Nigerien escort and have on the scene for less than 10 minutes.

The investigation into the ambush is being led by U.S. Army Major General Roger Cloutier, AFRICOM's chief of staff. A U.S. official said Cloutier was part of the team that visited Tongo Tongo on Sunday.

Last week, the Pentagon announced that as part of his investigation "Cloutier's staff will travel to locations in the U.S., Africa and Europe."

"AFRICOM is committed to carrying out a thorough and comprehensive investigation to ensure the families of the fallen understand what happened and how U.S. and Niger forces can learn from it," said the AFRICOM statement. "We will continue to pursue those who perpetrated this ambush.