Story highlights "We honor our troops, every one of them, every life is critical," Mattis added

Experts working for US Africa Command are trying to establish an hour-by-hour timeline of what happened

Washington (CNN) Defense Secretary James Mattis said Thursday that the "US military does not leave troops behind," but did not offer specific details about why the body of Sgt. La David Johnson was recovered nearly 48 hours after his 12-member team was ambushed by ISIS fighters in Niger two weeks ago.

Mattis also told reporters not to question the troops who were in the battle about how the soldier was separated and left behind.

"The US military does not leave our troops behind, and I would just ask you not question the actions of the troops who were caught in the firefight and question whether or not they did everything they could in order to bring everyone out at once," he said.

"We honor our troops, every one of them, every life is critical," he added.

Joint Staff Director Lt. Gen. Kenneth F. McKenzie told reporters Thursday that US, French and Nigerien forces "never left the battlefield" until Johnson was found.

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