Transcript for A look at how ISIS is recruiting young Americans through the internet
Reporter: This week in America -- There's been a violent -- Reporter: A 29-year-old man who emigrated to this country seven years ago takes a truck and mows down more than 20 people on a bike lane. On his phone, 90 ISIS propaganda videos. It seems for years now we have been talking about men like these, terror, radical jihad. But tonight in this report we are tracking something different. Trying to learn how ISIS is now reaching into all kinds of American homes. Muslim, nonmuslim, urban, rural. There are 1,000 terror investigations under way, reaching across every single state of the union. And 40% of those charged in supporting ISIS terrorism did not grow up Muslim. Which brings us to the young man who lived here in rural north Carolina. Police! What is your emergency? A desperate father called 911 about his son burning religious statues in their house. I need somebody as quick as possible. We have units en route to you. Stay on the line with me, okay? I don't know if it's ISIS or what -- This is your son? Yes. Reporter: You can hear that son in the background. They're going to throw me in jail my whole life and kill me. Why are you trying to say I'm a terrorist? Reporter: A terrorist? Justin Sullivan grew up as a boy in a catholic home with trophies, friends, a bedroom filled with childhood toys. His dad a retired marine captain rich Sullivan. I imagined him joining the military. And following my footsteps. Reporter: But his high school reported to the parents Justin had violent fantasies in his writing and suspended him. They said he needed serious help. The parents sent him to a psychologist but admit they downplayed what they were hearing. As Justin continued to isolate himself on his computer, obsessed with violent video games. And images of ISIS. ISIS soldiers. Who would wear their socks above their ankles. He started doing that himself. Reporter: The parents had no idea that their 18-year-old son, alone with his computer, had reached out to ISIS recruiters. He sent a message. In less than 24 hours, a top captain of ISIS responded. ISIS recruiters seemed to know exactly what to say to weaponize someone like Justin. There is even a captured terrorism recruiting manual. In phrases from other ISIS cases we can see step one. If the target is alienated like Justin, start with encouragement and praise. Like, welcome, brother. Blessings upon you. We know Justin loves the video game "Call of duty." ISIS has an idea, "Call of gee dahd jihad." Recruiters tell Justin there's something he can do to be a real action hero, a celebrity. One night in December 2014 in what seems to be a practice run, Justin puts on a ski mask and murders a neighbor with his father's rifle which he hides. Police find the neighbor's body but have no suspect. Then in a few months he tells his ISIS recruiter he's ready to go big. And ISIS says, make a videotape so they can be sure he's fame miss. Famous. He's going to get an assault rifle. And slaughter people at concert in North Carolina, or at a club, but as the clock is ticking down his parents discover a package containing a gun silencer. I called the police department, I need to talk to somebody in homeland security. Reporter: Captain Sullivan discovers the FBI was already secretly tracking his son because of that first call to 911. Agents descend on the house, arresting Justin. He pleads guilty to terrorism and to the neighbor's murder. His defense attorney argues Justin is clearly mentally ill. He could very well be on the brink of schizophrenia. Reporter: He has never talked about why he was trying to become a soldier of ISIS. Hello, this is a prepaid collect call from -- Justin Sullivan. -- An inmate at the county detention center. Reporter: He knows we've been asking the question. He says ISIS gave him a place to belong. I saw it as like a brotherhood. Reporter: His fantasies of heroism from the violent video games have become reality. It was like revenge. And you wanted to be like acknowledged by the islamic state. You want to be looked at as a hero. I wanted to be remembered. Reporter: At one point he claims he wouldn't really have gone through with a mass attack but we wondered does that mean he's sorry for what he did? Why should I apologize? I got two life sentences. Who'ser than me? Reporter: Mary Mac cord, former head of the federal office that has prosecuted all 147 ISIS cases in the U.S. In the last two and a half years. Including the case of Justin Sullivan. I don't think Justin Sullivan cared anything about the religion. He thought that this was essentially a death cult. Reporter: Mccord says it might surprise you how many of the cases her office has prosecuted have little to do with twisted religious beliefs, more to do with young men and 89% are male, just feeling powerless. For your teenagers it's just a few clicks away and most parents don't know. They have an entire chat room to give you information how to protect your computer from surveillance and the FBI. Reporter: Seamus Hughes of George Washington university leads a program and group that tracks and analyzes extremists online. They just posted this? They just posted this five minutes ago. Reporter: If you think smart, successful kids are somehow immune to the trap doors online, come with us to the federal transfer center in Oklahoma City. There is someone who can answer questions about the story of two honors students from small Mississippi towns. One of them a cheerleader, the other a soccer player. Through the door, Mohammad daklala, a young man who once had a bright future. Had you ever been arrested before? No, ma'am. Never before. Nothing? No, ma'am. Mean, unless you count like car tickets. One of your friends was quoted saying, he's the guy who was never in a bad mood. That is true. Yeah. Reporter: He is the son of a Muslim father and a catholic mother who converted. He was a straight-a student in graduate school, studying psychology, when he met his first real girlfriend. A superstar who grew up going to church on Sundays. Her dad a police officer, her mother a school principal. Super smart, super intelligent. Always very polite. Jalen delshon young. She was so smart, top of her class, studying to an doctor. Reporter: Mohammad shyly admits she was his first real relationship. What was it about her? Well -- looks, one thing. And the next thing to look at is intelligence, really. Reporter: So what happened? Jalen young has admitted in court that she was the one who became so unmoored, so overwhelmed with her premed studies, that anxiety and depression took hold. So she began to search for spiritual solace and looked at idealistic propaganda videos for ISIS. These videos are dangerously inspirational. If you're in a state of anger or state of depression, they really want to try to hook you in. Reporter: But it was Jalen herself who says she was the first one drawn to fundamentalist Islam. She like the strict rules of behavior. She says she became convinced by ISIS propaganda that ISIS was misrepresented in the media. In a surprise, the smiling former cheerleader began wearing an hikab. She tells her boyfriend she wants him to watch propaganda videos and come with her to live in Syria. He says, in truth, he just wanted to be with her. Are you saying you did this for love? Like blind love. But you knew. They're a savage organization. We took that to be like more of a biased view from the media. Did you really believe that? Look at your 4.0 average. I couldn't even think straight, to be honest with you. You were willing to kill Americans? No it was more just -- leek, you know, I'll want to help as much as I can. Reporter: He told me he hoped maybe he could work in the pr department. Then one day two years ago, after months of planning, these two college students packed their bags and headed to the regional airport to travel to the Middle East. What do they bring with them? Craft supplies, a scrapbook, a bar of soap, a pack of starburst minis, and his Nintendo. But it turns out their ISIS friends online were in fact undercover agents for the FBI, who moved in and arrested them. At sentencing, defense attorneys argued these were just smart kids who had fallen into depression, confusion, gotten lost in the internet. But the prosecutors argued, not so fast, that these were kids who knew what ISIS does and that they were prepared to join up. Both of them expressed deep remorse, but tonight Jalen young is spending 12 years in prison after prosecutors argue she instigated the idea. We asked to talk to her, she declined. Mohammad daklala is now in for for eight years. If you have a machine 24/7 whose whole purpose is to recruit any child is vulnerable. Any child, any religion? Anyone is vulnerable. The idea that you would go to join someone who engages in beheadings? That you would suspend critical thinking about that? They can dismiss the vile and disgusting things that they see, because somehow it all fits into the narrative. Reporter: Tonight, a father from North Carolina has a warning for parents. Remember, he is a marine who fought the enemy abroad. Here I am defending our country against domestic enemies. Which so happen to be my son. I didn't think it was this close to home. Reporter: A father awash in regret. I think we failed. Our thanks to Diane sawyer and her team for that compelling report.
This transcript has been automatically generated and may not be 100% accurate.