Good evening, not an. Reporter: Not an accident but an act of terrorism after a flight with 298 people on board was shot down over his country. How could this happen? Who was responsible? Why did they... See More
Good evening, not an. Reporter: Not an accident but an act of terrorism after a flight with 298 people on board was shot down over his country. How could this happen? Who was responsible? Why did they do it? Martha Raddatz brings us the very latest. Reporter: It's one of the last images of Malaysia airlines flight 17, posted on Facebook by a man believed to be on the plane. He included what turned out to be an eerie premonition. Should it disappear, this is what it looks like. Four hours into the flight, a burst of flame. This is the moment the plane crashed into eastern Ukraine. Smoke plumes fill the horizon. Those white streaks, debris from the aircraft littering the sky. As cameras reach the site of impact in the Ukrainian countryside, utter devastation, as far as the eye can see, an apocalyptic scene, leaving no hope that the 298 souls on board survived the fiery crash. But the debris was spread across ten miles, and in some spots, the pieces of wreckage showed no evidence of fire. It appears like the paint is fresh, but the metal itself has been ripped to shreds. What this could suggest is that the aircraft broke up in flight. Large pieces of plane with the Malaysian airline colors were still visible. In the rubble, clothing, passports, even a vacationer's travel guide. The part that continued on for ten miles is the part with fuel in it. Almost 20,000 gallons of fuel going up in smoke. Flight 17 took off from Amsterdam just after noon local time on its way to Kuala Lumpur, Malaysia. The plane crossed into Ukraine, nearing Russia's border. That's when something went terribly wrong. What probably happened, this crew was flying along on auto pilot, in the middle of the day, thinking everything was fine, all of a sudden there was a loud explosion and that was the end of that aircraft and all the lives on board. Reporter: Tonight the U.S. Believes that explosion was caused by a single surface-to-air missile. Shot down, not an accident. Blown out of the sky. They wouldn't have had a chance to outmaneuver it, once the missile impacted the aircraft, that flight was doomed. Reporter: It's unclear who filed the missile, but mh17 did fly over a war zone. The region below embroiled in conflict between the Russians and the Ukrainians. It's hard to understands why you would route an aircraft over a known combat zone. But the plane's altitude was determined to be safe for commercial air traffic, even though nato's top commander Philip breed love said recently that Russia has been providing air defense training to the pro-russian rebels. Why this Malaysian flight did not file to go left or right and not over the combat zone is still a mystery tonight. Reporter: On the ground, photographer and ABC news freelance producer John Wendel was one of the first journalists to arrive at the horrific crash scene. Narrating the scene to George stephanopoulos. You're at the crash site. Tell us what you see. It's a pretty remote country road and there are rebel fighters here who have secured the area. There's a smouldering pile of wreckage with kind of fire coming out of the top of it. I'm trying to head there right now. He was touring the wreckage with Russian separatist leaders now in control of the region. Who was talking to you there? Sorry, it's a rebel with a weapon. He just pointed out with a flashlight on the ground that there's -- there's blood splattered everywhere and pieces of -- pieces of remains -- oh, man. They're shining flashlights out into the field now. There are bodies scattered everywhere. They just shined a light on a woman, just a lower torso. Reporter: Since February, pro-russian separatists have been waging war against the Ukrainian government. They're fighting for Russia and for Russia's cause, and trying to create, in the eastern area of Ukraine, a separate nation that would then join up with Russia the way crimea broke away from Ukraine and joined up with Russia. A battle fought not only on the ground but in the skies. Just this week, it's believed Russian separatists shot down two Ukrainian military planes. But mh17 was a commercial flight, filled with civilians, not a military target. Almost immediately Ukrainian president poroshenko unequivocally placed the blame on Russia, which is accused of arming the separatists. Saying, it wasn't an accident or a disaster, but an act of terrorism. And Ukraine released what they say is an intercepted phone call, proving in the recordings, men they describe as militants talk about shooting down a plane. Later saying it was a civilian plane. I cannot imagine anybody on either side of this conflict wanting to take down a Malaysian aircraft. It was not an accident that they were trying to shoot down an aircraft, but they got the wrong aircraft. The pro-russian militants deny they are responsible. Somebody, whether it was the Russians or the rebels or somebody else, may have fired a radar-guided sa.-11 missile up to 33,000 feet, which impacted this airliner and brought it down, killing all aboard. Authorities believe a powerful missile launcher like this was used to shoot down the plane. So this was not just some small system. This was a very sophisticated surface-to-air missile. A system that requires extensive training and those who fired it may not have had enough. Perhaps what happened is that the rebels knew just enough to know how to launch one of these missiles, but not enough how to differentiate between a commercial aircraft and a military aircraft. Today president Obama offered his condolences to the families of the victims. The United States will offer any assistance we can to help determine what happened and why. And as a country, our thoughts and prayers are with all the families of the passengers, wherever they call home. Families of the victims gathered at Amsterdam's international airport and across the world in Kuala Lumpur, the flight's final destination. For the people of Malaysia, it's another devastating loss in less than five months. After Malaysia air 370 disappeared over the Indian ocean. It's a terrible coincidence for the people of Malaysia and for Malaysian airlines. That the aircraft that was shot down today was a Malaysian air triple 7, the same type and model as disappeared earlier in the year for reasons that we still don't know about. Along with the anguish, some staggering close calls. You get a sick feeling in the pit of your stomach. We were supposed to be on that flight. And obviously, you know, something watching over us and said no, don't get on that flight. But around the world tonight, thousands mourn and pay tribute to the 298 souls lost. For "Nightline," I'm Martha Raddatz in Washington.
This transcript has been automatically generated and may not be 100% accurate.