WASHINGTON — An emerging theory among U.S. military investigators is that the Army Special Forces soldiers ambushed in Niger were set up by terrorists, who were tipped off in advance about a meeting in a village sympathetic to local ISIS affiliates, three U.S. officials who have been briefed on the matter told NBC News.

The group of American Green Berets and support soldiers had requested a meeting with elders of a village that was seen as supportive of ISIS, and they attended the meeting at around 11 a.m. local time on Oct. 4, after a long night of patrolling, the officials said. Such meetings are a routine part of the Green Beret mission, but it wasn't clear whether this meeting was part of the unit's plan.

Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, did not address that theory when he briefed reporters on the incident Monday. He said the troops had been on a reconnaissance mission.

Three weeks after a deadly attack that has become a political flashpoint, the U.S. military is grappling to get a handle on the basic facts of what led to the deaths of four service members — and the growing chorus of questions about the U.S. mission in Niger and other parts of Africa.

Among the questions, Dunford said, was whether the mission changed; whether the intelligence was good enough, and why one of the fallen soldiers was separated from the rest of the unit.

"We owe you more information; more importantly, we owe the families of the fallen more information," Dunford said. "Did the mission change? It's a fair question."

Investigators are leaning toward a conclusion that local militants used the meeting in the village of Tongo Tongo to mount a sneak attack, officials said. Villagers sought to delay the troops as they tried to leave the village, according to officials. Once they departed, in unarmored vehicles, militants attacked them with small arms and machine-gun fire, the officials said.

Related: Deadly Ambush Highlights America's Growing Mission in Africa

The solders dismounted and began returning fire, and were soon facing mortars and rocket-propelled grenades launched from "technical" vehicles — light military vehicles — the officials said.

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The soldiers got back in their trucks and retreated about a mile before they were ambushed again. The attackers had trapped the Americans in a kill zone, the officials said, where they could envelop them in fire.

The two separate ambush sites could explain why Sgt. La David Johnson's body was found more than a mile from the coordinates from which the other dead and injured troops were evacuated by helicopter.

Army Sergeant La David T. Johnson Dept of Defense

The Americans didn't ask for help until about an hour into the firefight, the officials said. Once they did, a drone arrived within minutes, and French Mirage fighter jets arrived in about an hour, Dunford said.

"My judgment would be that that unit thought they could handle the situation without additional support," Dunford said, addressing the one-hour delay in the call for assistance.

The jets flew low over the site in an attempt to scare off the militants. The French jets did not drop bombs because the battlespace was so confused and they were not in radio contact with the Americans, the U.S. officials said. But the arrival of the jets did disperse the attackers, the officials said.

The mayor of the village told Voice of America, a U.S. government-sponsored news organization, that residents of the village sympathetic to militants delayed the Green Berets while the attackers assembled.

"The attackers, the bandits, the terrorists have never lacked accomplices among local populations," said Almou Hassane, the mayor of Tongo Tongo, in what the VOA said was his first interview with a Western news organization.

A map showing the location of Tongo Tongo, Niger. Google Maps

The village chief in Tongo Tongo, a man named Mounkaila Alassane, was arrested after the attack, Hassane said.

The latest information raises a fundamental set of questions, analysts said: Why would a small, lightly armed U.S. unit go into a village sympathetic to terrorists without drones overhead and a rescue force available if things went wrong?

"We know the proper way to do these missions so we can control risk," said one former special operator who works as a military contractor. "Every time you skip a step or use less resources, you incur more risk. They clearly skipped steps and had less resources than would be proper to see if they were walking into an ambush. At what level did someone accept the extra risk?"

Dunford spoke to the issue of too much risk, calling it "speculation," and said the investigation would get to the bottom of that question.

"Are they taking risks? They are," he said. "Are they taking risks that are unreasonable?...I don't have any reason to believe that."

In some ways the incident recalls what happened in December 2009 in Khost, Afghanistan, when an al Qaeda militant whom the CIA thought was going to give officers information instead detonated a suicide bomb, killing seven CIA officers and two other Americans. The CIA allowed the militant into its base without searching him because officers were trying to win his trust.

In this case, the Green Berets put themselves as risk seeking to win the trust of local residents, officials said.

There was no drone or other surveillance overhead when the attack commenced, but one official said there had been some overhead surveillance of the militants' preparation of the ambush — including men pushing motorcycles out of the village and then starting them out of earshot and driving them to the ambush site.

But those images were seen after the fact, the officials said, suggesting they may have been gathered by a sensor on a satellite.

Dunford said a drone was rushed to the area once the troops called for help, and it recorded video of some of the engagement. He has not seen the video, he said.

The Green Berets do not appear to have had any warning of a possible attack.