The Pentagon said Wednesday it has told family members of the four U.S. soldiers killed last month in Niger that its investigation won't be finished until at least January.

In a brief written statement, the Pentagon said Army officials contacted the families this week to give them an idea of when to expect answers to the many questions that have arisen since the soldiers were killed in an ambush Oct. 4.

Among the unanswered questions is whether the soldiers' commando unit was carrying out its assigned mission at the time it was ambushed about 200 kilometers (120 miles) north of Niamey, by what officials believe was a relatively new offshoot of the Islamic State group that calls itself Islamic State in the Sahel. Some reports have indicated the commando unit, which was operating with a larger team of Nigerien soldiers, might have been retasked at some point.

Also unclear is why one of the four Americans killed became separated from the others and why his body was not found until two days later. Others have raised questions about the adequacy of intelligence information available to the soldiers prior to the start of their mission.

The families were told the investigation is expected to be finished in January but could go longer.

An Army investigation team will travel to numerous locations in the U.S., Africa and Europe to gather information. The team is led by Army Maj. Gen. Roger Cloutier, the chief of staff at U.S. Africa Command.

The four killed were Staff Sgt. Bryan C. Black, Staff Sgt. Jeremiah W. Johnson, Staff Sgt. Dustin M. Wright and Sgt. La David T. Johnson.