The inconsistencies are at the heart of why the Pentagon has not been forthcoming with details about what happened in Niger, according to American military officials, who spoke on the condition of anonymity to discuss a continuing investigation. Four Americans were killed in the attack, including three Green Berets, as well as four Nigerien soldiers. Two Americans and six Nigeriens were wounded.
The contradictions added to the major questions emerging about the attack: Had the soldiers acted beyond their planned mission without first gaining approval? And if they were given permission, who granted it?
Military officials have said that the troops were on a reconnaissance patrol, which means they almost certainly were out to collect information on the Qaeda and Islamic State groups operating in the area; the American military has a list of Islamic State leaders they are targeting. For that mission, the commander of the American team would have needed approval from at least one or two higher levels — a subcommand in Chad and a task force commander in Germany, where the United States Africa Command is based.
Once in the field, if the team wanted to change the mission to pursue a suspected Qaeda or Islamic State leader, the team leader would need to conduct a risk assessment and call for permission from his higher headquarters, according to current and former senior officials at the Africa Command who described how its soldiers conduct operations.
The team would not have had “carte blanche to do whatever” it wanted, said Brig. Gen. Donald C. Bolduc, who until June commanded Special Operations forces on the continent.
Military officials said that the team members might have believed they were facing only a low risk of threats, which could have created a false sense of security that led them to go out with inadequate support.
Beyond the question of what, exactly, the soldiers were doing in that remote border region of Niger to begin with was that of whether Defense Department officials have been forthcoming about the mission.
Under the existing authorities, American ground forces are not allowed to conduct unilateral direct-action operations in Niger or most other countries in Africa, and the Pentagon continued to insist that it was not involved in combat operations there.
“Our missions are advise and assist,” Lt. Gen. Kenneth F. McKenzie Jr. told reporters on Thursday. “We’re not directly involved in combat operations.” He added: “We’re not involved in direct-action missions with partner forces.”
But if the American troops were giving chase to suspected Islamic insurgents to the Malian border, that would most likely qualify as a direct-action mission, military experts said.
What both American and Nigerien troops agree upon, officials from both nations said, is that on Oct. 4, what was supposed to be a routine patrol of a hostile border area in Niger crisscrossed by bands of armed terrorist groups — at least one of them loyal to the Islamic State — went wrong.
The soldiers had heard reports that a leader of the militant group had been coming from Mali to resupply on fuel and food at Tongo Tongo, whose villagers are seen as sympathetic to the extremists. The troops were anxious to follow up.
Suddenly, on the scrubby desert horizon, men on motorbikes appeared — a sign of likely terrorist activity. Motorbikes are the vehicle of choice of insurgent groups operating in the area, allowing them to easily navigate the harsh terrain, especially now, the end of the muddy rainy season. To crack down, the government of Niger recently banned the bikes in the area.
The American and Nigerien accounts differ on the next point: According to Nigerien officials, the convoy, described as a light patrol, pursued the men on motorcycles until they crossed into Mali. The 50 or so soldiers in the Nigerien-led convoy turned back and headed to Tongo Tongo to question village officials.
According to the American service members, they just saw the men on motorcycles nearby, but did not go after them, said Defense Department officials familiar with their version of events.
Both accounts then converge again, a Defense Department official said. The village chief in Tongo Tongo seemed to be stalling the service members, and military officials now believe he was allowing time for insurgents to assemble.
Eventually, after talking to the village chief, the troops got into their vehicles to return to their base, a two-hour drive. But less than five minutes after they drove out of the village, the convoy was ambushed by a group that outnumbered them two to one.
About 100 armed insurgents, many of whom were on motorbikes — two or three people a bike — as well as others in about 10 sport utility vehicles, surrounded the convoy. They were armed with heavy weapons, including antiaircraft weaponry as well as rocket-propelled grenades, according to a Nigerien official.
Soldiers in the joint patrol were riding in military vehicles as well as civilian Land Cruisers from the American Embassy. The firefight lasted two to three hours, the Nigerien official said, until a response unit from the military base arrived for reinforcements. French helicopters arrived to evacuate the dead and wounded as well as other soldiers.
Two or three vehicles in the convoy were destroyed in the firefight — most of the dead were in those vehicles. Part of the convoy became separated when at least one of the Land Cruisers became stuck in the mud. Sgt. La David T. Johnson, who died in the ambush, was in that vehicle, along with Nigerien soldiers who also died.
The village chief is in custody, the Nigerien official said. A few others are also in custody being questioned by intelligence officials. The F.B.I. is investigating the attack, an American law enforcement official said.
Both American and Nigerien military officials believe the insurgents were most likely affiliated with Adnan Abou Walid al-Sahraoui, who operates a group that has pledged loyalty to the Islamic State. They have yet to confirm that.
More than two weeks after the ambush, its repercussions showed no signs of abating. Senator John McCain, Republican of Arizona and chairman of the Senate Armed Services Committee, told reporters on Thursday that he might subpoena the Defense Department for answers. Defense Secretary Jim Mattis traveled to Capitol Hill on Friday to meet with Mr. McCain as well as another member of the committee, Senator Lindsey Graham, Republican of South Carolina.
The United States counterterrorism fight is “morphing” to places like Africa, Mr. Graham said after meeting with Mr. Mattis, adding that “we don’t want the next 9/11 to come from Niger.”