PENTAGON -- Details of the ambush in Niger are slowly emerging more than three weeks after four U.S. soldiers were killed. The Americans were part of a larger group looking for an Islamic militant, but his fighters found them first.

The patrol that was ambushed was tracking a terrorist, Adnan Abu Walid al Sahraoui, the leader of an Islamic State of Iraq and Syria (ISIS) offshoot that operates in western Niger.

On Thursday, the Pentagon acknowledged for the first time that there was a second team of U.S. and Nigerien forces was on the ground.

Lt. Gen. Frank McKenzie briefs reports on the ambush in Niger that killed 4 Americans. CBS News

"There was one that had something to do with this operation but I'm not going to be able to give you any more specific details about what happened until we complete the process of the investigation," said Lt. Gen. Frank McKenzie.

CBS News has learned that the second team was on a kill or capture mission for al Sahraoui. But it was called off after he slipped across the border into Mali.

Instead, the patrol with 12 Americans and 30 Nigeriens was told to investigate an area where the terrorist had been. The patrol was also told not to expect enemy contact.

After gathering intelligence, the men stopped in the village of Tongo Tongo. They were ambused by al Sahraoui's fighters as they left. Sources say the U.S. team did immediately alert their commanders they were taking fire. But the patrol waited an hour before asking for help.

French special operations forces were among the first to respond to the U.S. call for help during an ambush in Niger. CBS News

"You know, we look at that hour pretty hard ourselves, but again there are a lot of reasons why that time could've elapsed like that," McKenzie said.

Investigators still do not know how Sgt. La David Johnson got separated from the other soldiers during the attack. His body was recovered two days later.

CBS News learned on Thursday that a French special operations team was the first ground force to respond to the call for help from troops from the U.S.-Nigerien patrol when they were attacked on Oct. 4. The French team was on the scene within 3-4 hours of the call, after the French military sent jets from Niger's capital, Niamey, and Mali, a source said.

The U.S. military is still unsure how Army Sgt. La David Johnson became separated from forces on the ground in Niger. CBS News

Military officials briefed members of Congress Thursday. Democrat Richard Blumental said he had questions about the mission of the roughly 6,000 U.S. troops stationed in Africa.

"One of the reasons for this tragic catastrophe in Niger may well have been lacking resources in intelligence that could have permitted them to avoid that ambush," Blumenthal told CBS News.