The U.S. military has launched a formal investigation into an ambush in Niger that left four Army Green Berets dead and two injured, 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 confirmed Thursday.

"The loss of our troops is under investigation," Mattis told reporters. "We investigate anytime we have our troops killed, whether it be in a training accident or combat."

"These terrorists are conducting war on innocent people of all religions, they are conducting war on innocent people who have no way to defend themselves," he continued.

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"In this specific case, contact was considered unlikely, but there's a reason we have U.S. Army soldiers there and not the Peace Corps, because we carry guns," he said.

U.S. Africa Command sent a team to Niger to conduct a “review of the facts,” two U.S. defense officials told NBC News.

CNN reported that experts are trying to establish an hour-by-hour timeline of what happened. The investigation will include all U.S. military branches and agencies involved in the deadly mission.

During the Thursday White House press briefing, chief of staff John Kelly John Francis KellyMORE, a former general and Homeland Security secretary, said unless troops casualties are a "very conventional death in a conventional war, there's always an investigation," and nothing should be read into this one.

"An investigation doesn't mean anything was wrong, an investigation doesn't mean people's heads are going to roll. The fact is, they need to find out what happened and why it happened,” Kelly told reporters. “But at the end, ladies and gentlemen, you have to understand that these young people, and sometimes old guys, put on the uniform, go to where we send them to protect our country.”

The incident under review is the Oct. 4 ambush of a dozen U.S. soldiers from the 3rd Special Forces Group and 30 to 40 Nigerien troops during a routine train-and-assist mission. The group was attacked as they were leaving the village of Tongo Tongo along the border between Niger and Mali, about 120 miles north of the capital city of Niamey.

The U.S. soldiers were leaving in unarmored pickup trucks when roughly 50 Islamic State in Iraq and Syria-affiliated fighters fired on the group with small arms, machine guns and rocket-propelled grenades. The soldiers left the vehicles to run for cover and return fire.

The American and Nigerien forces were able to call in French air support 30 minutes into the attack, but the aircraft did not drop any munitions on the enemy fighters. Pentagon officials said last week they believe the air support was effective in ending the fight.

Staff Sgt. Bryan Black, Staff Sgt. Jeremiah Johnson and Staff Sgt. Dustin Wright died in the attack.

A fourth soldier, Sgt. La David Johnson, was first unaccounted for and a search-and-rescue operation with U.S., French and Nigerien troops was launched. Johnson’s body was later found and he was identified as a Green Beret killed in the attack.

Numerous questions remain after the incident, which occurred on a routine patrol that had been carried out 29 times in the past six months without incident, Joint Staff Director Lt. Gen. Kenneth McKenzie told reporters at the Pentagon.

The investigation will look into where U.S. forces were when the attack occurred, whether they had adequate personal protective equipment and whether they were prepared for the attack. The inquiry will also see if there was adequate intelligence in advance of the mission, if there was an adequate response to the attack and how Johnson was inadvertently left behind.

The Niger incident is the deadliest overseas attack on the U.S. military under President Trump, and has received considerable public attention since Monday, when Trump claimed incorrectly that former President Obama and other past presidents had not made calls to families of fallen soldiers. Trump made the remark after being asked why he had not yet addressed the four U.S. soldiers killed.

Since then, lawmakers have called for more information on the ambush.

Updated: 3:28 p.m.