Transcript for 4 Americans killed in Sri Lanka attack
And we begin tonight with new reporting here, inside that horrific terror attack. Multiple churches and hotels on Easter Sunday. The death toll is simply staggering tonight. Nearly 300 dead and there are several Americans among the dead, including a fifth grader. Just today, another explosion, a van bursting into flames fear one of those devastated churches. Sri Lankan TV showing this surveillance tonight. A man with a heavy backpack walking among a crowd of church Goers. Suddenly disappearing into that crowd. Among the Americans, I mentioned that 11-year-old boy that went to school in Washington, D.C. Another bomber waiting in line for brunch at a hotel. And tonight, we learn there were at least three warnings that there were possible plans for a terror attack under way. So, why wasn't there more security? ABC's James Longman is in Sri Lanka leading us off tonight. Reporter: Police eyeing this white fan when all of a sudden -- soldiers scatter, the neighborhood descends into chaos, yards from where we are. A huge explosion just went off just around that corner there. As soon as it did -- as soon as it did, buildings around here, windows shattered. A huge scream went off from the crowd over there. You can see them running towards our position where we are now. Police on high alert. We don't know what it was. We don't know if it was a police detonation, but that was absolutely terrifying. Terrified and bewildered as tonight, we learn that intelligence service ignored repeated warnings of imminent terrorist attacks they first received weeks ago. Then, on Easter morning, one after another, the suicide bombs went off. This dash cam video captures the first explosion at St. Anthony's catholic church. It was 8:45 A.M. The clock now frozen in time. Around that same time, a man with a backpack seen on security cameras approaching St. Sebastian's church in nearby negumbo. Investigators poring over these images broadcast on Sri Lankan TV, the man weaving through the crowd. Another camera inside the church showing him pass behind the worshippers. He is reportedly one of the bombers. And moments later, the weapon in the aftermath, the roof completely blown off. Where the faithful once net, only glass and debris. A celebration of new life now ravaged by death. But the attacks far from over. In the next 15 minutes, suicide terrorists strike three hotels popular with western tourists. One killer even waiting in line at a Easter brunch buffet before detonating his explosive. Later in the day, another hotel. 500 injured, victims rushed to hospitals. Outside St. Anthony's, bodies draped in white cloth, nearly 300 killed in these attacks, including at least four Americans. Dieter Kowalski of Colorado who was here on a business trip. And 11-year-old Kieran shafritz de zoysa who was to start seventh grade at Sidwell friends in Washington, D.C. His father remembering his son. A brilliant mind, who was going to be a neuroscientist. And now he won't make it to his 12th birthday. Reporter: 24 people are under arrest. Soldiers scaled the walls of this apartment building where they believe the attacks were planned. Moments later, another explosion. Three officers killed. Another very tense day there. James Longman joins us live tonight from Sri Lanka. And James, this evening, authorities are pointing to a local islamic extremist group for the attacks. Bull as you reported, authorities were warned ten days before the attacks, and people all over the world are wondering, why wasn't there more Reporter: People here are astounded at that, David, and authorities say this group must have had help from abroad, such was the level of sophistication and coordination of these attacks. It's possible they were inspired by ISIS, but the big question, was ISIS involved in carrying them out? David? James Longman on the scene again tonight for us. James, thank you.
This transcript has been automatically generated and may not be 100% accurate.