His killing is the latest violent death under mysterious circumstances for American troops on little-known missions in that region of Africa. Four American soldiers were killed in an ambush this month in neighboring Niger while conducting what was initially described as a reconnaissance patrol but was later changed to supporting a much more dangerous counterterrorism mission against Islamic militants in the area.

The Navy SEALs’ potential involvement also raised the prospect of a highly unusual killing of an American soldier by fellow troops, and threatened to stain SEAL Team 6, the famed counterterrorism unit that carried out the raid that killed Osama bin Laden.