New details from Niger ambush: when US troops sensed something wrong

— -- More than two weeks after four U.S. soldiers were killed during an ambush in Niger, new details about how the events unfolded have emerged.

A senior U.S. intelligence official familiar with the event tells ABC News that the U.S. troops saw early warning signs that something was wrong as they went to meet with the village elder in Tongo Tongo, Niger on October 4.

According to the official, suspicions were raised when the unit saw two motorcycles race out of the village.

"Hair on the back of their necks stood up," the official said.

At that moment, the patrol sensed the elder was trying to stall the group from leaving, the official said. When they finally departed, the enemy struck from both sides of a wooded road with fifty or more fighters that had smalls arms, vehicle mounted weapons, and mortars.

"This was sophisticated," the official continued, "Our guys not only got hit hard, but got hit in depth."

It is still unclear how many U.S. soldiers were part of the patrol, but the Pentagon has said it was between eight and twelve Americans.

With four killed and two wounded during the firefight, the official said the American unit has for now been “rendered combat ineffective."

Despite taking multiple casualties, the U.S. and Nigerien forces managed to kill at least twenty-one enemy fighters, who were later buried on the Malian side of the border, the official said.

"So while this was a tragedy, what's gotten lost is how well our people acquitted themselves," the official added, "We lost four but at least 21 from their side died."

The Defense Intelligence Agency has assessed that the ambush was perpetrated by ISIS in the Greater Sahara, an ISIS affiliate in West Africa.

It's still unclear why it took over 36 hours to recover Sgt. La David Johnson's body from the battle space. The official told ABC News that Johnson's locator beacon was giving unclear reports, and he seemed to be moving.

"Johnson's equipment might have been taken," the official said. "From what we now know, it didn't seem like he was kidnapped and killed. He was somehow physically removed from where the combat took place."

On Thursday, Joint Staff Director Lt. Gen. Kenneth McKenzie told reporters that Johnson was "separated" but "no one was left behind." That idea was echoed by Secretary of Defense James Mattis during a meeting with the Israeli Defense Minister.

"The U.S. military does not leave its troops behind, and I'd just ask not question the action of the troops that were put in the firefight and question," he said. "Don't question if they did everything they could in order to bring everyone else out at once and don't confuse your need for accurate information with our ability to provide it immediately in a situation like this."

It was nightfall when U.S., Nigerien, and French forces searched for Johnson.

The Pentagon has repeatedly said that the U.S. has conducted 29 patrols similar to the fateful one on October 4 without incident.