Transcript for ISIS leader al-Baghdadi dead after US raid in Syria: What we know
It's very terrifying here. Reporter: That's American Kayla Mueller in 2013. Held hostage by ISIS and brutalized by its leader. Al-baghdadi and his organization captured our daughter, held her for 18 months. They tortured her. She was held in solitary confinement. She was raped by al-baghdadi. We know that to be a fact. She was murdered by him or someone or someone in his organization. Reporter: Also among his victims, three other Americans captured in Syria. Aid worker Peter casic and journalist James Foley and Steven sotloff. Under baghdadi's control ISIS spread its gruesome campaign of terror on social media, to a degree never seen before. Barbaric beheadings, hostages burnt alive. But now a moment of justice after a U.S. Military operation killed Kayla's captor. The man who for many the world over was the face of evil, Abu Bakr al-baghdadi. On Saturday president trump with vice president pence gathered in the situation room with military brass, all watching a live step-by-step feed of the top secret mission. We had absolutely perfect, as though you were watching a movie. Reporter: Eight chinook helicopter took off from kurdish territory in Iraq, flying low and taking on gunfire before landing in northern Syria. Just after 5:00 P.M., 100 U.S. Delta force sdiers arrived at al-baghdadi's compound, where they blasted through a wall in case the front door was booby trapped. Al-baghdadi fled into an underground tunnel with three of his children. A military canine and soldiers followed in hot pursuit. He died after running into a dead end tunnel. Whimpering and crying and screaming all the way. He died like a dog. He died like a coward. Reporter: But before the special operations team could get to him, al-baghdadi detonated a suicide vest. The three children killed alongside him. His body was mutilated by the blast. The tunnel had caved in on it in addition. But test results gave totally positive identification. Reporter: Delta force operators were on the ground for roughly two hours in the firefight with baghdadi's men. At 7:15 P.M. After DNA testing on site, the call came in to the situation room from those on the ground saying 100% confirmation, jackpot. Over. We took highly sensitive material and information from the raid. Much having to do with ISIS. Origins, future plans, things that we very much want. Reporter: Today the president tweeting a photo of that heroic dog. I'm joined now in studio by defense secretary mark Esper. Great to have you here this morning. We have a lot to talk about. Reporter: My colleague Martha Raddatz was reporting key details this weekend as the story broke. This had been weeks in the making. Some time ago they had gotten a tip after the arrest of one of baghdadi's wives and a courier about where baghdadi might be. Reporter: Then spoke to defense secretary mark Esper about it Sunday. The U.S. Has been looking for him -- For years. For years. What was the break here? Good, very good intelligence work. They began tracking it for weeks and weeks. Of course in the middle of this we have U.S. Troops withdrawing, which a senior official told me made them have to do it now as soon as possible before our troops got out of there. Reporter: The world learned the name al-baghdadi in 2014, when he announced him as the leader of the islamic state, proclaiming his caliphate. When al-baghdadi was really known for was taking a terror group and actually gaining control of territory in Iraq and Syria and governing that territory, establishing what he called as a caliphate. Reporter: ISIS has inspired or claimed responsibility for horrific acts of terrorism here in America and around the world. There was the Bataclan concert hall and nearby cafes in Paris in 2015, where 130 innocent people were slaughtered. San Bernardino, California. 14 people shot to death at an office holiday party in 2015. 86 people were killed celebrating bastille day in nice in 2016 after a truck plowed into them. And in one of the world's deadliest terror attacks since 9/11, eight bombs killed 253 people on Easter Sunday in Sri Lanka earlier this year. Under baghdadi's leadership ISIS created a highly sophisticated, highly expansive online presence. They specifically targeted disaffected people across the west, and they sought to inspire these people to on behalf of the ISIS cause but independent of the organization itself. Reporter: Around the world and here at home the threat of ISIS is real and ongoing. This year ISIS suspects were the targets in 1,00 of the 5,000 active FBI terror investigations. It is very easy to get drawn into continued conflict if our objectives are not clear. Reporter: Meanwhile, officials saying U.S. Forces will continue their counterterrorism mission in Syria. The world of terrorism or end the ongoing conflict in Syria. But it will certainly send a message to those who would question America's resolve and provide a warning to terrorists who think they can hide. Reporter: An effort that relies heavily on allies. This weekend's raid was launched from kurdish territory in Iraq. But earlier this month president trump had pulled U.S. Troops out of northern Syria. As the U.S. Reduces the number of military and intelligence resources in the region, counterterrorism officials have to be concerned that that will greatly diminish our ability to collect intelligence. Reporter: The administration's decision drew criticism from Republican and democratic lawmakers alike. Who said America was abandoning its kurdish allies, one of the most reliable partners in the war against ISIS. It's always a good day when a terrorist leader like al-baghdadi dies. But historic tells us a new leader will emerge. The group will reorganize itself. And the key question here, will the U.S. Be in a position to address the next leader? Reporter: Families of Americans killed by ISIS say they are thankful that al-baghdadi is gone. The parents of James Foley speaking to ABC affiliate WMUR. We're grateful that our country has recognized the need to continue to find the leadership of ISIS. While the victory will not bring our beloved son Steven back to us, the significant -- it is a significant step in the campaign against ISIS. Reporter: The muellers also hope that they will learn more about their daughter Kayla's fate. Someone that they've captured in that group that was close to al-baghdadi knows what happened to her. All along my only real goal was to bring Kayla home. Back to Arizona, where she grew up and was born. She never gave up hope, though, from what we were told. She never gave up hope. On us getting her home. Reporter: For "Nightline" I'm Pierre Thomas in Washington, Our thanks to Pierre.
This transcript has been automatically generated and may not be 100% accurate.