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The warning signs were there.

In the days leading up to the deadliest school shooting of 2021 in the U.S., 15-year-old Ethan Crumbley made a —both online and . He had been caught at school . The drawing on his desk, , depicted a gun pointing at the words "The thoughts won't stop. Help me."

It is, perhaps, easy to look back at the postings of a mass shooter after the event and highlight the red flags that were potentially missed.

But how do you know when a young person is writing offensive, threatening or disturbing posts merely to garner attention or to blow off steam, rather than presenting a threat to themselves or others? And at what point in the transition from online threats to real-world harm should concerns by teachers, parents or peers be deemed actionable by law enforcement and other officials?

As experts , we believe many of the investigative tactics used in countering terrorism can be leveraged to prevent violent acts like the one Ethan Crumbley is charged with carrying out. In particular, monitoring social media and systematically assessing the threat of individuals who post disturbing content can prove critically important.

Among the charges Crumbley faces is , a cases. Karen McDonald, the prosecutor in Oakland County, Michigan, spoke of the similarity between and terrorism: "If that's not terrorism, I don't know what is."

As such, which lessons can be used from terrorism research to help identify potential lone-actor shooters?

They leak information about the attack

on lone-actor terrorism indicates most "leak" crucial information before an attack. In up to 74% of , such individuals shared crucial details of the planned attack with friends, family members or co-workers. The study looked at lone actors across a variety of movements, from far-right nationalists to Islamist extremists to single-issue groups, such as the Animal Liberation Front. In some of these cases, details were posted online before the attack.

They display disturbing behavior

Speaking on CNN, Candice Delong, a active shooters often exhibit aggressive and inappropriate behavior. Indeed, prosecutor McDonald " that Crumbley planned the attack in advance, adding that there was "additional evidence that hasn't been released yet."

Investigators are continuing to review Crumbley's social media and online behavior—not only what he posted but also his search history. At his arraignment, materials recovered from Crumbley's cellphone included "a video made by him the night before the incident wherein he talked about shooting and killing students the next day at Oxford High School." "Further, a journal was recovered from Ethan's backpack also detailing his desire to shoot up a school to include murdering students."

Our own assessment of what is publicly known about this material suggests striking similarities between Crumbley's behaviors before the shooting and the types of radical online content—manifested in memes, images and gaming platforms—most often associated with violent extremist groups that target and recruit young people.

They radicalize online first

Our finds that extremists rely heavily on social media and the internet to recruit and access vulnerable . And like individuals recruited online, school shooters can often experience a similar transition—they go from posting violent fantasies online to engaging in real-world actions.

In Crumbley's case, he moved very quickly from online posts to real-world harm, than most lone-actor shooters who can take several months or even a year before they manifest violence.

They draw inspiration from online violence

Critically, it is not necessary for extremist groups to . Many lone-actor shooters perpetrate acts of violence through mere exposure to such messaging, especially when it resonates with a young person's existing insecurities or grievances.

This creates a pernicious form of plausible deniability—where extremist groups or individuals can simultaneously incite violence through multiple means and deny responsibility for the actions of individuals who are inspired by them. Psychologist Randy Borum has argued multiple motivations might be the norm, but online incitement is certainly a common theme.

They fixate on perceived enemies and injustices

Lone-actor shooters can become focused on imagined

This behavior has been evident in the past few years with "incels"—involuntary celibates—who blame women for their social rejection and use violence to express their misogyny. According to the U.N. Office of Counter-Terrorism, in their manifestos, journals and other writings.

We do not yet know the specific motivations behind the latest school shooting. But we do get a picture of Crumbley as someone acting out of perceived . And in this way, he may not be dissimilar from mass killers like , , or who identify or have been identified as incels.

The common theme is these young men's frustrations, along with growing feelings of , is that they transitioned from being expressed through online fantasy to real-world tragedy.

Crumbley matches many of the stereotypical characteristics associated with other mass shooters: He was young, white, male and seemingly disaffected. It is also clear that Crumbley was attracted to firearms.

Reading the signs

Not all young men posting and consuming violent content online will carry out acts of violence. The majority, in fact, won't. But experience shows that some are at risk for taking what they see online to try to make it happen in the real world, and the consequences of these few acts can be devastating to families and communites.

In Crumbley's case, warning signs appear to have been missed. School officials did meet with Crumbley and his parents over his behavior twice, including on the day of the attack, but as Tim Throne, the district superintendent, , "At no time did counselors believe the student might harm others."

School lockdowns, active-shooter drills, and campus security——cannot replace preemptive prevention. Identifying and getting to would-be shooters before they can move from the hypothetical online world to the physical real world is critical.

Reading those signals and setting up early-warning interventions before they turn into a tragedy is critical work for those in public safety and academia.

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