French air support arrived two hours after Islamic State-aligned fighters began a deadly ambush on U.S. Green Beret advisers in Niger, according to a new timeline provided Monday by the chairman of the Joint Chiefs of Staff.

However, it was still unclear why the team of 12 U.S. soldiers and their partner force of 30 Nigeriens waited a full hour after the firefight began to request backup from French Mirage fighter jets, or why the body of one soldier, Sgt. La David Johnson, was found two days later by the Nigeriens, Gen. Joseph Dunford said during a press conference at the Pentagon.

"What tactical instructions the commander on the scene gave at a given time that caused units to maneuver and where they might have been when Sgt. Johnson's body was found, those are all questions we'll identify during the investigation," he said.

The press conference was meant to provide a "baseline" of what the Pentagon knows, Dunford said. U.S. Africa Command continues to investigate the Oct. 4 incident and questions swirl on Capitol Hill over whether the military is being transparent.

Defense Secretary Jim Mattis traveled to the Senate on Friday to meet with Sen. John McCain, R-Ariz., and pledged better communication after the Armed Services chairman said his committee was not getting answers and might consider subpoenas.

Dunford said the soldiers and their Nigerien counterparts were returning to base the morning after a reconnaissance mission to a village north of the country's capital when they came under attack by about 50 ISIS fighters.

One hour later the soldiers requested backup and within minutes a nearby U.S. drone was diverted to provide "full motion video … right over the scene of the troops in contact," Dunford said.

The French, who have 4,000 troops in the region, were also notified at the same time and scrambled the Mirage fighter jets within 30 minutes, which took another 30 minutes to arrive at the scene of the firefight, he said. By the afternoon, there were also French attack helicopters on the scene.

Two wounded U.S. soldiers were flown out by the French during the fight, which was part of the mission plans for dealing with casualties, and the three who were killed were evacuated that evening when Johnson was determined to be missing, Dunford said.

"Many of you have asked a number of questions and many of them are fair questions and we owe you more information," he said. "More importantly, we owe the families of the fallen more information. That's what the investigation is designed to identify."

How Johnson was separated is among the questions that the military is still pursuing, as well as whether the U.S. troops properly assessed the danger of the mission and whether they had adequate equipment and training, Dunford said.

He gave no timeline for completion of the investigation.

The U.S. has 800 troops deployed to Niger and a total of 6,000 in countries across Africa to train and assist local forces in combatting Islamic State and al Qaeda influence.

Dunford said there are currently no discussions of increasing those operations but some lawmakers, such as Sen. Lindsey Graham, R-S.C., have said the war against the Islamic State in Iraq and Syria may be shifting to Africa and more aggressive missions may be necessary.