The Defense Department is conducting a review of the mission in Niger and the ambush by 50 ISIS-affiliated fighters two weeks ago, which left four US soldiers dead and two wounded, CNN reported on Tuesday.
The investigation is seeking "to get all the facts correct" on what was apparently a confused situation during the unexpected firefight, officials said.
Soldiers who were on the ground are being interviewed about what happened, as well as preparations for the mission.
Although preliminary findings are expected by the end of the week, they are likely to remain classified. It is still unclear how Congress will be briefed about the incident.
Senate Armed Service Chairman John McCain said on Tuesday that the Trump administration has not provided enough information on the attack in Niger, The Hill reported.
"I had a better working relationship, as far as information back and forth, with Ash Carter [defense secretary for the Obama administration] than I do with an old friend of 20 years," said McCain, referring to Defense Secretary James Mattis, as well as national security adviser H.R. McMaster.
"I think they had this idea that once Trump won that we are a unicameral government," said McCain, a Republican senator from Arizona.
Among the key questions being asked in the Defense Department investigation are why did the intelligence indicate it was "unlikely" the 12-man team led by Green Berets would run into opposition, after multiple visits to the area; why was there an attack at this time; and once the ambush broke out, how did the body of Sgt. La David Johnson get left behind and was he ever alive or was he killed instantly?