The U.S. military is still searching for answers on what happened in Niger two weeks ago when four U.S. soldiers were killed during an ambush, apparently by a branch of ISIS.

Now the Pentagon's Africa Command (AFRICOM) has sent a team to the African nation to conduct a "review of the facts," according to two U.S. defense officials. The officials are careful not to call the inquiry an investigation, but admit they simply don't know what happened on Oct. 4.

"We need to collect some very basic raw facts," one defense official said.

In addition to the Pentagon, a top Senate Republican wants answers. Armed Services Committee Chairman Sen. John McCain of Arizona told reporters this week that the Trump administration was not being forthcoming about what happened in Niger.

"I want the information that the Senate Armed Services Committee deserves and needs," he said.

Some of the facts the AFRICOM team needs to collect, said one defense official, are: Where were U.S. forces when the attack occurred? Did they have adequate personal protective equipment and were they prepared for the attack? Was there adequate intelligence in advance of the mission and adequate response to the attack?

Chadian troops and Nigerian special forces participate in exercises with the U.S. military and its Western partners in Mao, Chad, in March 2015. Jerome Delay / AP file

The official said the level of confusion during and after the mission was "tremendous." The fourth soldier's body wasn't found until nearly two days after the ambush.

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Pentagon officials say operations in the region have already "tightened up" and there's been an operational "pause" while AFRICOM assesses the situation.

U.S. officials say the ambush was likely carried out by an ISIS affiliate called ISIS in the Greater Sahara. ISIS-GS is just one of several ISIS affiliates operating in the region.

The attack "has not been claimed by a terrorist group, but a group claiming association with ISIS, ISIS in the Greater Sahara, is likely responsible," said a US official. A second U.S. official called the group more ISIS "wannabe" than a direct affiliate of the Syrian-based terror group. However, said the official, it would be wrong to suggest ISIS-GS is unsophisticated.

Sgt. La David Johnson was killed in an ambush in Niger. U.S. Army Special Operations via AP

The attack and its aftermath have become a political issue in recent days with President Trump under criticism for not initially calling the families of the four killed, for suggesting that prior presidents had not called the families of the fallen, and then for the alleged content of his call to the widow of one of the four soldiers.

The four Americans were killed Oct. 4 when a 12-man team of soldiers from the 3rd Special Forces Group was operating with approximately 30 Forces Armees Nigeriennes (FAN) on a train-and-advise mission near Tongo Tongo, Niger, just miles from the Mali border. The patrol was seen as routine and in fact had been carried out 29 times in the six months before the ambush, the Pentagon has reported.

Militants, both Al Qaeda in the Islamic Maghreb (AQIM) and ISIS, have been using a nearby route to travel back and forth into Mali and back to a base camp in Niger, the official said. The partner forces were working to disrupt the so-called rat line and interdict the militants.

The 12 U.S. soldiers went to the village to get supplies — food and water. As part of the mission, they visited village elders. Special Forces soldiers often conduct "key leader engagements," making contact with local leaders.

The team was split up, with eight in the key leader engagement and the other four with the vehicles.

At the conclusion of the meeting, as the Americans were leaving, they were attacked by 40-50 militants with RPGs and AK-47s. The attackers were "well-equipped and trained," one Pentagon official said.

The U.S. returned fire but the officials still do not know how many militants may have been wounded or killed in the firefight. There was a platoon of Niger soldiers nearby but U.S. defense officials cannot confirm reports that any of them were wounded or killed.

Armed French Mirage fighter-bombers arrived on scene within about 30 minutes. The planes didn't drop any bombs or fire on the attackers, but U.S. defense officials believe their presence helped break up the firefight.

One indication of the level of confusion after the attack is that the U.S. military has provided three different answers for who flew the medevac helicopter – first U.S. military officials said it was French military, then that it was the U.S. military. Now, they're saying it could have been a U.S. contractor.

A U.S. Army carry team transfers the remains of Army Staff Sgt. Dustin Wright of Lyons, Georgia, at Dover Air Force Base in Delaware on Oct. 5, 2017. Wright was killed in Niger. Aaron J. Jenne / U.S. Air Force via Reuters

If, as officials believe, ISIS-GS carried out the attack, it would be the group's first against American forces. In the past, they have carried out attacks on French counterterrorism forces.

ISIS-GS was established in 2015 after the group's current leader, Adnan Abu Walid al-Sahrawi, broke from al-Murabitun (an al Qaeda-associated group) and pledged allegiance to ISIS leader Abu Bakr al-Baghdadi and ISIS. U.S. officials said the group usually uses small arms and mortars to conduct ambushes and complex attacks.

ISIS-GS has not been formally recognized as an official branch of ISIS but its pledge of allegiance to the terror group was acknowledged by ISIS leadership in Syria in October 2016.

The U.S. has five outposts in Niger as well as a military presence at the international airport in the capital of Niamey, which is in the far west of the country. The U.S. also has a drone base in Agadez in Central Niger from which the U.S. can monitor militant activity as far north as Libya and as far south as Nigeria.