"The US military does not leave its troops behind and I would just ask that you not question the actions of the troops who were caught in the firefight and question whether or not they did everything they could in order to bring everyone out at once," he told reporters.
Joint Staff Director Lt. Gen. Kenneth F. McKenzie told reporters Thursday that US, French and Nigerien forces "never left the battlefield" until Johnson was found.
Three senior US officials told CNN on Wednesday that Mattis wants answers regarding the ambush in Niger two weeks ago.
Mattis is dismayed at the lack of detailed information he has received about about the attack, but there is no indication he is trying to unduly hurry the investigation being carried out by US Africa Command, according to all three officials -- all of whom are in a position to have knowledge of how Mattis views the situation.
"This was a hard fight, this was a very tough fight," Mattis told reporters last week -- providing little detail about what multiple US officials have described to CNN as a scene of confusion on the ground during an unexpected firefight.
The investigation will be an effort "to get all the facts correct," an administration official familiar with the review has told CNN.
White House spokeswoman Sarah Sanders was asked Wednesday whether Trump was satisfied with the information he has received about the mission and ambush.
"I believe they're still looking into the details of that," Sanders replied. "But I don't think that the President can ever be satisfied when there's loss of life from men and women in uniform."
What we know
Details related to the deadly military breakdown remain murky two weeks after the incident in Niger as investigators work to determine precisely what happened, a US official has told CNN.
Experts working for Africa Command are trying to establish an hour-by-hour timeline of what happened as part of a comprehensive investigation that includes all the military branches and elements of US intelligence agencies that were involved in the mission.
Officials who have spoken to CNN have cautioned that as the investigation continues new information is likely to emerge and their understanding of what happened could change.
Very little has been said publicly, but the information that has emerged in the wake of the attack paints a troubling picture of what transpired.
Here is what we do know:
Four US soldiers were killed and two wounded: In what is the deadliest combat mission of Trump's short presidency to date, the Defense Department has identified all four service members killed in the ambush that occurred near the Niger-Mali border by up to 50 fighters from ISIS in the Greater Sahara, a US official said.
Sgt. La David Johnson, Staff Sgt. Bryan Black, Staff Sgt. Jeremiah Johnson and Staff Sgt. Dustin Wright died as a result of the October 4 attack, after helping local forces in Niger combat terrorists.
Initial reports suggested the 12-member US team was leaving a meeting in unarmored pick-up trucks when they began taking fire from small arms, machine guns and rocket-propelled grenades, according to a US defense official.
With window glass exploding all around them, the service members, including multiple Army Special Forces soldiers, exited the vehicles, ran for cover, and began returning fire, killing some of the attacking militants.
But later officials said the 12-man Green Beret-led team were actually not in the trucks having just completed a meeting with local leaders and were walking back to the unarmored pick-up trucks when the unexpected ambush resulted in a firefight that lasted 30 minutes until French Mirage jets arrived overhead to fly low passes in an attempt to disperse the attackers.
Sgt. La David Johnson was separated: A large-scale search-and-rescue operation involving US, French and Nigerien troops was launched soon after US officials realized one of the US service members was unaccounted for.
Johnson was later identified as the fourth service member killed in the attack.
His body was recovered in a remote area of the northwestern African country by Nigerien troops nearly 48 hours after he was discovered missing in the wake of the attack, according to US officials.
US, French and Nigerien forces engaged in the search operation remained on the battlefield until Johnson was found, McKenzie told reporters on Thursday, emphasizing the fact that he was separated and not left behind.
"The search continued until he was found. We never left the battlefield until he was found," he said.
Johnson's body was returned to Dover Air Force Base on October 7.
Intel rated it "unlikely" US team would face opposition: One official has told CNN that the military's intelligence said it was "unlikely" that the team would run into enemy forces.
"This was not expected," US Africa Command spokesman Army Col. Mark Cheadle said.
"Had we anticipated this sort of attack we would have absolutely devoted more resources to it to reduce the risk and that's something we are looking at right now," he added.
ISIS affiliate likely responsible: CNN has reported that 50 fighters of the regional ISIS in the Greater Sahara were responsible for the attack.
The Pentagon has claimed the group emerged in Niger because of defeats suffered by ISIS in Syria and Iraq.
Sen. John McCain was pressed Wednesday on whether the administration was being up front about the ISIS-affiliated attack, McCain answered bluntly: "No."
"We do have information on the group
that did it, their nature, their disposition and so on and so forth and appropriate organizations within the United States military are digging deeper into that and will take appropriate action if required," Army Chief of Staff, Gen. Mark Milley, told reporters last week.
There are about 800 US troops in Niger and the US military has maintained a presence in the northwest African country for five years, with small groups of US Special Operations Forces advising local troops as they battle terrorist groups, including ISIS in the Greater Sahara, the ISIS-affiliated Boko Haram.
Private contractor conducted evacuations:
A US private aviation contractor conducted evacuations
of US and Nigerien troops after they were ambushed, according to US Africa Command spokesperson Robyn Mack.
Mack said that US private contractor Berry Aviation was "on alert during the incident and conducted casualty evacuation and transport for US and partner forces."
US officials previously told CNN that French military Super Puma helicopters also evacuated the wounded Americans along with those killed in action while also providing covering fire. The wounded were first flown to the capital Niamey and later to Landstuhl Regional Medical Center in Germany.
McCain wants answers: Sen. John McCain said Thursday that he might seek a subpoena in order to receive the information he wants surrounding the attack.
"It may require a subpoena," McCain told CNN's Suzanne Malveaux on Capitol Hill when asked what steps he would be willing to take to receive the information he's demanding on the attack.
McCain argued Wednesday that the Trump administration is not being forthcoming about the attack in Niger.
Asked if he thinks Congress should launch an investigation into the attack, the chairman of the Senate Armed Services Committee told reporters that first he would like to get the information that his panel "deserves and needs."
"Then you decide whether a quote investigation is needed or not," he said.
The Arizona Republican did not go into detail about what kind of information he was looking for, saying only that he was interested in "all the specifics."
Calls for Benghazi-like investigation: The incident is already raising the prospect of a Benghazi-like investigation similar to when Congress looked into the attack an on American diplomatic compound in Libya that resulted in four US deaths in 2012.
Asked about the prospect of hearings similar to those following the attack in Benghazi, Sen. Mike Rounds said: "In the Benghazi incident you had a case of where there was clear testimony information coming out saying there were hours of activity going on. If similar facts were to be determined in this particular case, you may very well see the same type of a demand for a review."
What we don't know (but the military might)
Missed indicators that ISIS was operating in the area?: CNN has previously reported that the fighters who carried out the attack were part of an ISIS-affiliated group called ISIS in the Greater Sahara but the Trump administration is yet to mention ISIS as the responsible party.
Officials familiar with the initial after-action reports say there was confusion and uncertainty on the ground after what was a completely unexpected attack. The team was particularly vulnerable because it was in two separate locations when the attack began. Some were walking back from a meeting with local villagers. Others were waiting outside, guarding the vehicles that the US troops were using.
US officials assess that terrorist groups view Chad, Niger and Mali as being particularity important as they serve as bridges between north and sub-Saharan Africa, saying that local al Qaeda and ISIS affiliates use control of these transit routes to gain revenue that helps them recruit, expand and export attacks.
ISIS uses these North-South transit routes to move fighters northward, where they can more easily access Europe and the West.
One official told CNN that ISIS is attempting to illegally infiltrate the gold mining industry in Niger to sell on the black market and finance world terrorism.
Why did it take so long for Trump to mention the casualties?:
Perhaps wary of the political ramifications President Donald Trump has still not publicly stated
that an ISIS affiliated group was responsible.
White House officials prepared and circulated internally a statement
to be issued on the President's behalf the day after the attack. But the statement, obtained Wednesday by Politico and confirmed by CNN, was never officially released.
It wasn't until nearly two weeks later that the President, an avid Twitter user, mentioned the deaths when he asked about them during an impromptu news conference Monday in the White House Rose Garden.
Commenting for the first time Monday on the attack, Trump said he had written letters to those families over the weekend and they were being sent out that night.
The ambush occurred amid news that ISIS self-declared capital in Raqqa, Syria was on the verge of collapse meaning Trump may have been inclined not highlight an attack showing ISIS's staying power abroad.
There is also the issue that Trump allowed US troops to be in a country that does not allow offensive air operations.
While French Mirage jets arrived overhead within 30 minutes of the firefight to fly low passes in an attempt to disperse the attackers, the aircraft did not have permission to drop bombs, multiple US officials told CNN.
Trump has received criticism for his long silence following the attack and handling of the aftermath -- particularly his public feud
with Rep. Frederica Wilson, a Florida Democrat, over Trump's phone call with Myeshia Johnson, the widow of Sgt. La David Johnson.
After multiple visits to the area, why was there an attack at this time?: Last week, Joint Staff director Lt. Gen. Kenneth Mckenzie said the patrol that was attacked had completed 29 patrols in the area without contact with hostile fighters over the previous six months or so and there was no indication an attack would occur on October 4.
How did Sgt. La David Johnson become separated?: It is unclear how Johnson became separated from the rest of the advisory team during the firefight. None of the other soldiers with Johnson witnessed him being captured or taken away by enemy forces.
The US military initially said it did not believe Johnson ever fell into enemy hands and had reason to believe he might be alive. Military officials launched an urgent search-and-rescue mission, but officials now say initial reports they received electronic signals that indicated Johnson might have been alive in the field were inaccurate.
While French helicopters were able to get the team to safety, the critical failure to find Johnson for another 48 hours has not been explained.
His body was eventually found in a nearby area, but military investigators do not know why he was separated and if he was alive even for a short period of time, US officials tell CNN.
Questions remain about how and when Johnson was killed.
Were aircraft contractors in full communication with the French?: While CNN has reported that both a private contractor aircraft and French military helicopters took part in the evacuation mission it remains unclear if both parties shared all necessary information related to the operation.
"The wounded were airlifted out by the French and the service members killed in action were evacuated by contract air," the Pentagon told reporters on Thursday.
The failure to anticipate an attack and the fact there were no US rescue and recover assets close by meant nearly an hour went by before the evacuation of the two wounded and three dead US troops by French helicopters could be completed.
Mattis said the rescue was timely stating: "I completely reject the idea that that was slow." But he did say an investigation will determine if changes are needed. "We will look at this and say was there something we have to adapt to now? Should we have been in a better stance."
UPDATE: This piece has been updated to include Thursday's Pentagon briefing, comments from Sen. McCain, Secretary Mattis and CNN's latest reporting on what happened in Niger.