So far, no one has been charged in Sergeant Melgar’s death, which a military medical examiner last year ruled “a homicide by asphyxiation,” or strangulation.

The Naval Criminal Investigative Service took over the case in September 2017 from Army criminal authorities after the status of the two Navy commandos was changed from “witnesses” to “persons of interest.” That meant officials sought to determine what the commandos knew about the death and whether they were involved in its cause.

The SEALs were in Mali on a clandestine mission to support French and Malian counterterrorism forces battling Al Qaeda’s branch in North and West Africa, known as Al Qaeda in the Islamic Maghreb, as well as smaller cells aligned with Al Qaeda or the Islamic State.

The Americans helped provide intelligence for missions, and had participated in at least two such operations in Mali before Sergeant Melgar’s death. About two dozen American troops operate in Mali at any given time, mostly to help on training and counterterrorism missions.

SEAL Team 6, formally known as the Naval Special Warfare Development Group, has over the past decade carried out kill-or-capture missions in Afghanistan, Libya, Iraq, Somalia and Yemen, as well as the one that killed Osama bin Laden in Abbottabad, Pakistan, in 2011.

According to the preliminary Army report, one of the SEAL members put Sergeant Melgar in a chokehold. When the sergeant passed out, the commandos frantically tried to revive him — going so far as to perform CPR and a field-expedient emergency tracheotomy. When that failed, they rushed him to an emergency clinic, where he was pronounced dead.

Officials have said that one service member who knew Sergeant Melgar said that the sergeant’s chain of command immediately grew suspicious when the initial incident reports said the death was the result of a drunken accident. His friends and superiors knew that Sergeant Melgar did not drink.