WASHINGTON -- For the first time, we are seeing images of one of four U.S. soldiers killed by Islamic militants last October in the African nation of Niger. The video was captured by a National Geographic cameraman for a report airing Monday.

Coming just one month before his death, the small talk in a makeshift barber shop between Sgt. La David Johnson and one of his commanders is especially poignant.

Army Sgt. La David Johnson was killed in an attack in Niger CBS News

"Finding stuff to do like making sure my truck's good, making my generator's good," he said. "Like regular maintenance on the day."

Johnson was the mechanic for a special forces team that trained Nigerien soldiers and, according to chairman of the Joint Chiefs of Staff Gen. Joseph Dunford, accompanied them on low-risk missions.

"They were authorized to accompany Nigerien forces when the prospect of enemy contact was unlikely," Dunford said.

The enemy, an offshoot of al Qaeda, was described to American commanders by the chief of Niger's special operations forces.

"These guys are very mobile," the chief said. "They move on motorcycles, land cruisers."

The special forces set out on a routine patrol but then were given a second mission to check out a camp used by a suspected terrorist who had fled across the border into Mali. That change of plans significantly increased the chance of enemy contact -- even if the terrorist was no longer there.

Sgt. La David Johnson went on a second mission CBS News

Retired Brig. Gen. Donald Bolduc is the former commander of special operations in Africa.

"I would still consider it a very high risk mission in that there is still intelligence that isn't exactly definitive on exactly what the enemy situation is there," Bolduc said.

As they returned from that mission, they stopped at a village for water. Shortly afterwards, they were ambushed and the four soldiers were killed. Johnson's body was found two days later, riddled with bullets.

Which commanders approved that more dangerous mission and why is part of an investigation that was supposed to be completed in January but has only now reached the secretary of defense.