Amid growing political fallout, the Department of Defense has put forward a timeline for the deadly confrontation with militants in Niger that killed four U.S. troops.

Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, said during a briefing that major questions remain about this "tough firefight," such as whether the unit's mission changed and why it took days to recover the body of Army Sgt. La David Johnson. He also said investigators are examining whether the troops had adequate intelligence, equipment and training.

But despite those questions, he sketched out the following timeline for how the unit came under fire from what Dunford described as local tribal fighters associated with ISIS while the team was heading back to base with Nigerien forces:

"Early morning of 3 Oct. ... U.S. forces accompanied that Nigerien unit on a reconnaissance mission to gather information. The assessment from our leaders on the ground at that time was that contact with the enemy was unlikely."Mid-morning on Oct. 4, the patrol began to take fire as they were returning to their operating base. Approximately one hour after taking fire, the team requested support. And within minutes, a remotely piloted aircraft arrived overhead. Within an hour, French Mirage jets arrived on station. And then later that afternoon, French attack helicopters arrived on station and a Nigerien quick reaction force arrived in the area where our troops were in contact with the enemy."During a firefight, two U.S. soldiers were wounded and evacuated by French air to Niamey. And that was consistent with the casualty evacuation plan that was in place for this particular operation. Three U.S. soldiers who were killed in action were evacuated on the evening of 4 Oct., and at that time Sgt. La David Johnson was still missing. On the evening of 6 Oct., Sgt. Johnson's body was found and subsequently evacuated."

It's not clear how, exactly, Johnson was separated from the rest of the group and where his body was found in relation to the firefight.

Another major question is why the group confronted the militants for a full hour before there was any indication of their calling for external support.

"I make no judgment about how long it took them to ask for support. I don't know that they thought they needed support prior to that time," Dunford said. "I don't know how this attack unfolded. I don't know what their initial assessment was of what they were confronted with."

Dunford said that according to U.S. policy in the area, U.S. military personnel do not accompany local troops if contact with militants is seen as likely. That means they were not expecting resistance, he said. Five Nigerien partner troops were also killed in the confrontation.

The incident has raised questions from lawmakers about U.S. military involvement in Africa, with senators including Lindsay Graham expressing surprise at the troop levels in Niger.

On NBC's Meet the Press, Graham defended comments from Sen. John McCain, saying, "This is an endless war without boundaries, no limitation on time and geography. You've got to tell us more. And [McCain is] right to say that."

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