It remains unclear how Johnson, of Miami Gardens, Fla., became separated from other U.S. troops on the patrol or if he had fallen into enemy hands. Officials said Saturday that the incident is under investigation.
On Friday, U.S. officials identified three of the fallen soldiers as Staff Sgt. Bryan Black, 35, of Puyallup, Wash.; Staff Sgt. Jeremiah Johnson, 39, of Springboro, Ohio; and Staff Sgt. Dustin Wright, 29, of Lyons, Ga. All four were assigned to the Army's 3rd Special Forces Group, which is based at Fort Bragg in North Carolina and serves as the U.S. military's primary unconventional-warfare unit operating in Africa.
Four Nigerien troops were killed and two other U.S. troops were wounded in the battle, but their names have not been disclosed. Officials said the Americans were in stable condition at a U.S. military hospital in Germany.
The French military, which also maintains a presence in the region, scrambled Mirage fighter jets in response to the attack, presumably to search for La David Johnson and provide support for those under threat on the ground.
The White House on Friday said President Trump spoke with French President Emmanuel Macron to discuss the situation in Niger. Press secretary Sarah Huckabee Sanders demurred when asked whether the administration intends to avenge the troops' deaths.
"Obviously," she replied, "any time one of the members of our great military are injured, wounded or killed in action, that is something we take very seriously. . . . We're continuing to review and look into this," she added, noting that White House Chief of Staff John F. Kelly, a retired general whose Marine son was killed in combat, had kept President Trump apprised of efforts to locate the missing service member.
Trump met with his top generals and admirals Thursday to discuss a range of national security issues. While posing for a group photo, he cryptically referred to the moment as "the calm before the storm." When asked what he meant, the president responded, "You'll find out."
The Pentagon and the White House have long sought to frame the U.S. military's activities in Niger and elsewhere in Africa as providing support for American allies battling extremists throughout the region — and being removed from direct combat with those groups.
Officials have said the attack occurred during an "advise and assist" mission, a broad term that critics say downplays the danger associated with training partner nations using small numbers of troops near militant strongholds.
U.S. forces have expanded efforts in Niger, military officials have said, as part of a growing presence in the Sahel region. The vast expanse of desert stretches across the continent, and affiliates of al-Qaeda and the Islamic State have taken advantage of instability in Libya, where arms and fighters flow into a region difficult to govern.
About 800 U.S. personnel are assigned to posts in Niger, mostly at two sites focused on gathering aerial reconnaissance for Nigerien forces. That is an increase from 645 in June. About 300 to the south in Cameroon provide logistical and intelligence support. An unknown but probably small number operate in Mali.
The four combat deaths mark the first known hostile-fire casualties among U.S. forces in Niger. A soldier with the 3rd Special Forces Group was killed in a vehicle accident there in February.