Mr. Shanahan did not give a timeline for his review, saying only that it would end soon.

In May, United States Africa Command finished an initial investigation into the ambush and found widespread problems across the military operation. That inquiry focused mostly on the actions of junior officers before an hourslong gun battle against dozens of Islamic State militants.

A separate investigation, by Special Operations Command, has been in limbo as high-ranking Defense Department officials have been divided on its outcome. It largely sought to overhaul training protocols but also designated soldiers for awards for valor and for punishments.

In December, before he left the Pentagon, Mr. Mattis criticized Defense Department officials for the length of the investigation and the fact that most of the resulting punishments were issued against junior officers with relatively little experience.

The Army has since punished officers and enlisted soldiers who were involved in the battle at nearly every level of command, like a captain, who was the Green Beret team leader in the ambush, and the two-star general who oversaw all Special Operations Forces in Africa. The punishments focused primarily on training failures that led up to the attack on Oct. 4, 2017.

Col. Bradley D. Moses, then the commander of the Third Special Forces Group, is the one of the only people in the Special Operations chain of command involved in the ambush who remains unpunished.