Transcript for New details on Niger ambush that killed 4 US soldiers

The latest out of Washington. President trump is heading to capitol hill this afternoon to meet with Republican senators and make a push on tax reform. He's promising thought to make changes to 401(k) Plans. And there are new developments in the Russia investigation. Sources tell ABC news that the president's longtime personal attorney will also be on capitol hill today for a closed door interview by the house intelligence committee. And serious questions are growing about that deadly ambush that killed four special forces troops. Our chief global affairs correspondent Martha Raddatz is in Washington with new reporting on this. Good morning, Martha. Reporter: Good morning, robin. It is a shock to many that the U.S. Has some 800 troops serving in Niger, most at a drone base but nearly 100 there to assist Niger's military in countering ISIS and Al Qaeda and it is ISIS, the military believes, that killed those four Americans. This morning, chilling new details about that the ambush that left four American special operations soldiers dead in Niger. A senior intelligence official telling ABC news it started as a routine reconnaissance mission, a convoy of six to eight vehicles, three American, traveling to a local village. But after they completed that mission, a new order came through, a dramatic change in plans, the team now told to kill or capture one of the top ISIS targets in Niger. They never find the target. But on their way back to base after being awake for nearly 24 hours, the group stopping in a remote village, a survivor of the ambush telling ABC news something is off. A village elder tries to stall them. And leaving the town, the attack, 50 terrorists are upon them. The team taking fire from rocket propelled grenades and machine guns. One disabled by gunfire. U.S. Forces waiting for more than an hour to call for help. Within minutes a U.S. Drone overhead but it appears the troops waited about an hour before calling in French warplanes that arrived an hour later. In about two hours after the initial contact was made, the initial French mirages arrived overhead. Reporter: As night falls two wounded U.S. Troops are evacuated. So are the bodies of the three U.S. Soldiers killed in action. But sergeant la David Johnson is still missing. Why did it take time to find and recover sergeant Johnson? Again, these are all fair questions that the investigation is designed to identify. Reporter: His body would not be recovered for another 48 hours. But that survivor telling ABC news sergeant Johnson was, quote, grabbing every weapon available to him to fight and was, quote, a beast. His widow speaking on "Gma" to George yesterday demanding answers. I don't know how he got killed, where he got killed or anything. I don't know that part. They never told me and that's what I've been trying to find out since day one. Reporter: General Dunford says the Pentagon is working hard to get those answers for Mrs. Johnson and the other families, one major question, why would those special operations soldiers be sent on a new dangerous mission without backup.

This transcript has been automatically generated and may not be 100% accurate.