PENTAGON -- Army Sgt. La David Johnson was one of four American soldiers ambushed and killed in Niger, where the U.S. is assisting in the country's fight against terrorism.

Just over two weeks later, the circumstances are far from clear, and now the FBI is joining the investigation.

Johnson's body was found nearly a mile away from the ambush site. Why he got separated from the other Americans and how long he survived remain unclear.

A rendering of what possibly happened in the ambush in Niger. CBS News

Pentagon officials say a locator beacon was tracked for several hours, fueling hopes he was alive, and raising the agonizing question of whether he could have been saved if the U.S. military had been able to react faster.

But the biggest mystery is why the patrol had not expected enemy contact in a part of Niger, where a U.N. map shows 46 attacks took place over the last 20 months.

The initial explanation by the Pentagon is that the team of special forces advisers had conducted nearly 30 patrols with Nigerien troops and not contact with the enemy once.

The patrol had no armed air cover and the one reconnaissance aircraft flying at the time was over another part of the country. When the patrol radioed for help, French Mirage jets arrived 30 minutes later, making low-level runs which scared the attackers away -- but by then three soldiers were dead, two wounded and one missing. A French helicopter medevaced the wounded. A helo flown by an American contractor came in to pick up the dead.

Secretary of Defense James Mattis on Capitol Hill. CBS News

Defense Secretary James Mattis went to Capitol Hill on Friday to brief Sen. John McCain on what went wrong, but the chairman of the Armed Services Committee already had part of the answer. Gen. Thomas Waldhauser, the commander of U.S. forces in Africa, told Congress in March he had only a quarter of the reconnaissance flights he needed and had to rely on contractors for search and rescue missions.

Waldhauser said providing search and rescue to troops in the field is a moral obligation. So the investigation has to determine not only what happened on the ground in Niger but why Washington failed its obligation to the troops.