Five years ago, a deadly attack during the Boston Marathon made America’s nightmare come true: the radicalized boy next door.

The research my colleagues and I conduct at Georgia State University tracks how terrorist organizations expose people – mostly young men – to radical messages and extreme violence on social media. The goal: changing their worldview and eventually guiding them to act.

The Boston Marathon bombing marked the beginning of a new trend that is almost impossible to prevent. Before, individuals would receive guidance and training from terrorist organizations in person. Now, these same groups simply inspire individuals to carry out attacks on their own, for which the group can claim credit if they are successful. We call that “self-radicalizing.”

Radicalization of the boy next door

It remains puzzling to many how Dzhokhar Tsarnaev, a 19-year-old stoner who listened to Jay-Z and watched “The Walking Dead,” could – with his older brother, Tamerlan – killed and injured so many innocent civilians. Authorities at least knew more about Dzhokhar because he was taken alive. His brother died during a police chase.

Dzhokhar did not fit the profile experts have identified as typical of self-radicalized terrorists – either jihadi (a Muslim Holy warrior) or extreme right-wing versions. They were unlike many terrorists who gravitate to extremist ideologies.

According to researchers Alice Marwick and Becca Lewis, people who are radicalized “may have a hard time finding like-minded friends in their day-to-day lives, or connecting with romantic partners.”

Dzhokhar was described by Rolling Stone magazine, for example, as attractive, popular and a student athlete. He was also flunking courses and lost his financial aid at University of Massachusetts-Dartmouth in the semester leading up to the attack. But that hardly explains why he went on to plan the deaths of potentially hundreds of runners and spectators.

Eventually, prosecutors found copies of Inspire magazine on Dzhokhar’s laptop. The magazine is an English-language online publication that was published by al-Qaida in the Arabian Peninsula. They also found videos of sermons by Anwar al-Awlaki, the American-born firebrand jihadi cleric who was killed in 2011 in Yemen by two U.S. drones. Awlaki’s videos are still circulating on social media years after his death.

The evidence and Dzhokhar’s testimony suggest that the brothers were inspired by propaganda. Both Awlaki’s sermons and Inspire magazine advocate and provide specific “how-to” instructions on mass casualty attacks. Dzhokhar and his brother learned how to make the pressure-cooker bombs from one of the most well-known articles published by the magazine: “How to Build a Bomb in the Kitchen of Your Mom.”

For example, the text explains: “Can I make an effective bomb that causes damage to the enemy from ingredients available in any kitchen in the world? The answer is yes. But before how, we ask why? It is because Allah says … every Muslim is required to defend his religion and his nation.”

In the five years since the Boston bombing, the number of social media platforms disseminating terrorist propaganda has increased tenfold. And thus, so has the scale and scope of possible future attacks.

Shift to encrypted platforms

As technology evolves, new online platforms provide avenues for terrorist organizations to share information. Platforms have gone from an open interface model, in which anyone can observe what is occurring, to closed and encrypted platforms in which privacy and security settings are protected. These platforms are not readily accessible nor can they be viewed without already being a member of the chat room or channel. When used by terrorist groups, encrypted platforms are harder to police and monitor.

Telegram is an encrypted application developed by Pavel Durov, the inventor of Russia’s Facebook equivalent, VKontakte. Telegram has all but replaced Twitter and Facebook for jihadi communication. Open platforms like Twitter and Facebook have increasingly policed jihadi content, shut down their sites, and taken down their content within minutes of it being posted.

Social media allows terrorist groups to foster a virtual community and a sense of belonging. Research on radicalization suggests that their methods involve taking advantage of individuals’ feelings of loneliness and alienation. However, this doesn’t explain why well-adjusted, well-integrated individuals who appear to have assimilated into Western society – like the Tsarnaevs – gravitate to violent extremist ideologies.

Research I’ve done with colleagues on social media demonstrates that terrorist organizations also deliberately foster a type of addiction to the platform and to its content.

They do this by creating a schedule of positive reinforcement that modifies behavior, like gambling or playing slot machines. The result changes every time such that one cannot anticipate the outcome and continues to engage in behavior in hopes of a reward the next time around. Not knowing whether you are on the verge of a big win sustains the individual to continue the behavior (in some cases, gambling), whereas not knowing what type of reward the terrorist platform will offer equally sustains user engagement.

This keeps the individual tethered to his or her computer, making certain rewards more valuable, limiting how much time you have to join a group or access material, making access exclusive, and varying content to prevent boredom.

Through this process, it is possible that ordinarily well-adjusted individuals can be persuaded to channel everyday feelings of frustration, disappointment or anger into acts of extreme violence.

As the Islamic state further recedes and loses even more territory, it is adapting to exist almost entirely digitally. Those who fight terrorism need to understand the difference between individuals who are inspired and those who are directly encouraged to engage in violence. Likewise, terrorism researchers need to to understand the role of social media in capturing the imagination of young men and instigating them to perpetrate violence.

Mia Bloom receives funding from the Minerva Research initiative Documenting the Virtual Caliphate Minerva #N00014-16-1-3174 and the office of naval research. All opinions are exclusively those of the author and do not represent the Department of Defense or the Navy.