The U.S. special forces patrol caught in a deadly ambush in Niger was on a kill or capture mission instead of a reconnaissance mission, as U.S. officials first described, ABC News reported Thursday.

Four senior Nigerien officials told ABC that last month's mission — which led to the deaths of four Army Green Berets — was conducted without additional support requested by Nigerien forces, 30 of which accompanied 12 U.S. troops.

The American team leader expressed unease over the mission, after a second U.S. and Nigerien group was unable to join them, a senior U.S. intelligence source told ABC.

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Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE on Monday told lawmakers that the troops involved in the ambush were on a train and advise mission. Most of the 1,000 U.S. troops are in the region for intelligence, surveillance and reconnaissance, as well as logistics such as refueling, he said.

Gen. Joseph Dunford, the chairman of the Joint Chiefs of Staff, meanwhile, said last week that the American and Nigerien soldiers were on a “reconnaissance mission,” with the U.S. in an “advise and assist” role, when they were ambushed by about 50 ISIS-linked fighters near the border with Mali.

But Nigerien sources said the mission was organized and led by Americans and meant to kill or capture a top terrorist leader in the country known as Dandou and code-named “Naylor Road” by the U.S.

Two U.S. intelligence sources told ABC News the team’s mission morphed halfway through, and they were first asked to meet local leaders in the village of Tongo Tongo. The team was additionally assigned the Naylor Road mission on their way back to base, meaning they were out for more than 24 hours.

“They should have been up and back in a day,” a senior U.S. intelligence official told ABC. “But they were up there so long on a mission that morphed, they were spotted, surveilled and ultimately hit.”

After they left Tongo Tongo and were en route back to their base, the team came under attack by fighters with machine guns, mortars and rocket-propelled grenades.

Pentagon officials confirmed last week that a second U.S. military team was in the same area, but would not give additional details.

ABC also reported a Nigerien military operation is being planned against those that initiated the attack and other attacks in the area. The operation — which will have U.S. support — was said to be accelerated because of the deaths of the American soldiers.