The killing of three men at a San Diego mosque on Monday is the latest example of a disturbing trend in recent decades: hate-motivated shooters learning from – and copying – each other in acts of violence meant to push the nation toward a race war and, ultimately, societal collapse.

The two San Diego shooters, who were 17 and 18, killed 51-year-old Amin Abdullah, a security guard at the Islamic Center of San Diego, 78-year-old Mansour Kaziha, a mosque elder and founding member of the center, and Nadir Awad, 57, who lived across the street and whose wife worked as a teacher at the center’s school.

The shooters later died of self-inflicted gunshot wounds.

The pair appears to have been deeplyentrenched in online extremist networks and looked up to shooters who have killed dozens of people in US places of worship, schools and grocery stores. In a 75-page document the shooters wrote before Monday’s attack, they expressed hatred for Muslim and Jewish people, for Black people, the LGBTQ+ community, women, and for both political parties in the US.

In addition to the document spelling out their extremist views, they also livestreamed their violence and wrote on their firearms in white marker – all common tactics among shooters who have been radicalized online, said Matthew Kriner, executive director of the Institute for Countering Digital Extremism, which has been investigating the shooters’ digital footprint to understand a possible motive for the attack.

“Preliminarily, we’re seeing two individuals who jointly radicalized into this digital space and then jointly radicalized into this moment of violence,” Kriner said.

In a Tuesday briefing, JD Vance, the vice-president, decried the shooting as “reprehensible”.

“The principle of religious violence is particularly disgusting, especially in the United States of America,” he said. “And as a devout Christian, I would say it’s one of the most anti-Christian things and anti-American things that you could do.”

After the killing of Charlie Kirk in Utah in September, and assassination attempts on Donald Trump’s life, the president, his administration and their allies have said rhetoric from Democrats and the left is catalyzing political and extremist violence.

Earlier this month, the White House released its counter-terrorism strategy, which said the biggest threats of domestic terror and violence come from “Narcoterrorists and Transnational Gangs, Legacy Islamist Terrorists and Violent Left-Wing Extremists, including Anarchists and Anti-Fascists”.

Experts say there is a growing body of research showing that recent high-profile shootings in the US are mostly committed by people heavily influenced by online spaces, where traditional political divides are largely irrelevant.

The counter-terrorism strategy also made no mention of white supremacist violence or young people who are being radicalized online, even though federal officials have previously acknowledged the risks these networks pose.

In the past year, researchers and the FBI have also warned of an increase in nihilistic violent extremism (NVE), which the federal government describes as violence “motivated by a hatred of society and a drive to cause its collapse through indiscriminate chaos”.

It’s unclear how many high-profile shootings are tied to this specific brand of hate. But in February the FBI said it was investigating at least 350 people across the US who it believes are connected to NVE networks that are extorting vulnerable young people into acts of terrorism and sexual violence.

Last March, the FBI released a notice warning against groups that radicalize young people toward hate-motivated violence and use“threats, blackmail, and manipulation” to coerce theminto acts of violence against themselves or the public. And in March this year, the US attorney for the central district of Illinois issued a press release about how these groups were continuing to use “digital platforms to exploit and radicalize children”.

In their manifesto, the San Diego shooters both frequently refer to the man who shot and killed 51 people at two mosques in Christchurch in New Zealand in 2019. Kriner said thisshows that many violent extremists are not tied to conventional political beliefs, but instead are adopting the ideology of other shooters before them.

The 23-year-old who shot and killed 32 people at Virginia Tech in 2007 was inspired by the two Columbine high school shooters. Nearly a decade later in 2016, authorities found a book about the Virginia Tech and Columbine shooters in the bedroom of an 18-year-old German man who shot and killed nine people at a Munich shopping mall.

The 18-year-old who shot and killed 10 Black people in a Buffalo grocery store in 2022 had been inspired by the Christchurch shooter three years prior, as well as the shooters at an El Paso, Walmart and a historically Black church in Charleston, South Carolina.

The gunman who killed two children at the Annunciation Catholic school in Minneapolis last August expressed admiration for the 28-year-old shooter who killed three children and three adults at a school in Nashville, two years before.

Kriner said most high-profile extremist shooters were not constrained by traditional political binaries such as Democrat or Republican. Instead, their views are a mashup of fascist ideals and a feeling that the world has fallen into such a state of disrepair that extreme violent action must be taken.

“The core belief of doing something, taking action, appeals across social, racial, economic and national lines,” Kriner said. “It makes it hard for us to find the motivating factor because everything becomes a factor.”