Johnson fought valiantly against overwhelming enemy forces but were ultimately killed during the October 4, 2017 ambush in Tongo Tongo, a military spokesman told reporters on Thursday.
The Pentagon said the dead United States troops fought bravely and "died with honor" but investigators found they had not been properly trained and that only half of the American team had trained together.
While the Pentagon said all four USA soldiers fought bravely and "died with honor", investigators found they had not been properly trained ahead of the mission and pointed to problems with how it was approved in the first place.
At least five Nigerien soldiers also were killed, and other soldiers were wounded, including two Americans.
The summary details the events that led up to a massive firefight with USA and Nigerian forces heavily outnumbered by enemies associated with the Islamic State. La David Johnson and two Nigerien soldiers got separated from the others during the battle and were gunned down, but his body wasn't found until two days later.
As a result of the incident, the U.S. military has ordered a review of training for special forces soldiers working with foreign partners and policies on pre-deployment training, and U.S. Africa Command is seeking to create clear guidance on the planning and approval of military operations.
When the ISIS militant was not found, troops were ultimately redirected to a mission to gather intelligence on the militant, which they completed before stopping near the village of Tongo Tongo to get water.
Additionally, the Pentagon is holding out the option of disciplinary action for some of the missteps that contributed to the ambush and the loss of four USA and four Nigerien soldiers.
"The whole thing was a screwed-up mess", said Arnold Wright, father of Staff Sgt. Dustin Wright of Lyons, Georgia, who was killed in the attack.
In a briefing shortly after the ambush, Joint Chiefs Chairman Gen. Joseph Dunford said the mission had been expected to pose little risk.
Cloutier described a chaotic firefight, as 46 US and Nigerien forces battled greater than 100 enemy fighters for greater than an hour. Gen. Roger L. Cloutier Jr., chief of staff for U.S. Africa Command and the lead investigating officer of the incident.
During the crossfire, the caravan was broken up and the three vehicles operated by US soldiers were separated. They were driven off by French aircraft.
The bodies of two of the soldiers were found in the back of abandoned pickup truck, with a third body lying alongside it on the ground. No U.S. or Nigerien forces were harmed in the December gunbattle, which the Pentagon first acknowledged in March in a terse line in an unclassified report to Congress.
Johnson's body was found after all the others, and he had run almost 1,000 meters away from the attack site to try to save himself under intense fire. None were captured alive by the enemy, and all died immediately or quickly from their wounds, it said. They told the Washington Post in November that his hands appeared to have been tied behind his back.
"The evidence would indicate that up until that point, they believed it was a small enemy force that they believed they could handle", said Army Maj.
The combat - along with at least 10 other previously unreported attacks on USA troops in West Africa from 2015 to 2017 - indicated that the deadly October 4 ambush was not an isolated episode in a country where the United States is building a major drone base.
The investigation has already triggered changes in the way military activities are carried out in Niger and elsewhere in Africa, including giving teams the option to use heavily armored vehicles and beefed-up firepower.
The episode ignited a larger political controversy after a Democratic congresswoman reported that President Trump had upset the widow of one of the slain soldiers in a condolence call, saying the president had stumbled over the name of the soldier, La David Johnson, and suggested he "must have known what he signed up for".