Tonight, new details emerging about a popular teen that opened fire in his school cafeteria. Killing one student and injured four others before killing himself. What made a homecoming prince into a... See More
Tonight, new details emerging about a popular teen that opened fire in his school cafeteria. Killing one student and injured four others before killing himself. What made a homecoming prince into a cold-blooded killer? Neal Karlinsky has the story. Reporter: Children running from what should be a safe place. This time, in Marysville, Seattle. Early this morning, the first call comes in from inside the school. A now sickeningly familiar alert. Five to six shot. I counted five, but I think there was six. Reporter: Emergency responders who had just practiced for something like this quickly swarmed the area. We need a gurney here. Reporter: Police checking locked doors with guns drawn. Minutes later, the gunman reported dead from a self-inflicted gunshot wound. And another four injured, three in critical condition. All of them inside the school's cafeteria. I thought it was firecrackers, and three more went off, people screaming, run, it's a gun! I fell over. Got up, there was someone laying on the ground. Reporter: This boy lifts his shirt to show where a shot gra grazed his back. I heard a shot go off, everyone ran out, and I heard some gunshots. Reporter: You heard the shots? Soon, parents flooding the area. Going to a nearby church to be reunited. We're lucky. She called me right away so I didn't have to worry for too long. Thank god, I can't imagine the parents that hadn't heard from their kids, standing over there. It's emotional, everywhere you look. You can see the buses that had taken the kids from the school. It's one huge mass of parents and kids. Reporter: The alleged shooter, not an outcast or loner, but this year's homecoming prince. Jaylen fryberg, a popular student. He had just returned to school after reportedly being suspended for a fight. I see him every day. Reporter: Was he mad about something? No, he was fine. Reporter: Can you make any sense of it? You know him. I don't know, like, I have no clue. Because he was fine the day before. He was being sassy as always and good. Reporter: Another friend says he spoke to him earlier today. From what I heard, there was a couple of bullying incidents. Reporter: So, what could have set him off. Twitter offers some clues, Jaylen writing, it won't last. Also earlier writing, you were right. It breaks me. And on instagram, this photo. He and his birthday present, a gun. Though not believed to be the murder weapon. Boys feeling some sense of failure or loss. Reporter: Dave Cullen says the repeat pattern in so many of them is depression. If we can attack that problem, we can cure the problem before it gets to the point of a gun. Reporter: School shootings have become painfully routine around the country. Tonight, the shooter's family is keeping to themselves. But the chairman of the native American tribe they belong to had this to say. Sadly, we all know what has become a national trend, and we as a society must address this issue. Reporter: Tonight, the school's championship football game cancelled. And the district plans to close school all of next week. We pray for the families that you would comfort them, god, tonight. Reporter: And a community came together to show support for the victims, and the families that lost so much, and to try to understand something that makes to sense. Neal Karlinsky, for "Nightline," Washington.
This transcript has been automatically generated and may not be 100% accurate.