Inside the chilling cult of the 'Columbiners': Disturbing number of school shooters around the world inspired by 1999 bloodbath as it's linked to latest tragedy where Natalie Rupnow, 15, killed teacher and student

On April 20, 1999, two teenage boys dressed in black trench coats went on a killing rampage at Columbine High School in what was at one time the deadliest school shooting in American history.

Armed with an assortment of weapons and homemade bombs, Eric Harris and Dylan Klebold shot and killed 12 classmates and a teacher at the Colorado school, and wounded two dozen others before taking their own lives.

Twenty-five years later, the name Columbine has become synonymous with school and mass shootings in America and across the world.

The massacre has spawned a subculture of fans, known as Columbiners, who are fascinated with the killers and have created an online community with an estimated more than 70,000 members worldwide.

Harris and Klebold’s infamous slaying has created a school shooting blueprint for 'copycat' shooters and been linked to more than 50 fatal hate-filled attacks across the globe including in Russia, Germany, Finland and Canada.

But the Columbine effect has seemingly had the worst impact on America with 32 fatal school shootings having been connected to the attack, including the three deadliest school massacres in US history.

Virginia Tech shooter Seung-Hui Cho said he wanted to 'repeat Columbine' before carrying out his 2007 attack. Sandy Hook gunman Adam Lanza had an 'obsession' with Columbine and Parkland shooter Nikolas Cruz researched the 1999 massacre before gunning down 17 people at his Florida high school.

There have been 426 school shootings - not all of which were fatal - across the US since the Columbine massacre, according to The Washington Post's School Shooting tracker, with the most recent occurring just three days ago.

Natalie Rupnow, 15, armed herself with a pistol and opened fire at her small Christian school in Madison, Wisconsin on Monday, killing a teacher and another student. Six others were injured in the attack.

Rupnow, who turned the gun on herself after the massacre, has been pictured wearing the same band t-shirt as Columbine killer Harris.

Although it was not the world's first-ever school shooting, Columbine was the first infamous school massacre because people were intrigued by its unique circumstances.

Columbine, unlike other school slayings, was carried out by two gunmen who, in hopes that their story would outlive them, created home videos of the preparations for their attack and detailed their plans in a written manifesto.

The pair also did not target anyone in particular when they stormed the storm, but instead fired randomly at their victims. They had also designed bombs that were intended to kill indiscriminately. 

Harris and Klebold's attack came at the start of the digital age and the 24-hour cable news cycle, coverage about the violent attack was readily and permanently available to anyone across the globe.

The first so-called Columbiners were mostly just 'curious teenagers' fascinated by the criminal mind and interested in analyzing the massacre. 

But now the online cult is comprised of thousands of adolescents who openly say they condone the Columbine murderers and have even likened Harris and Klebold into heroes, according to the Atlantic.

Columbine groupies, who according to the magazine often describe themselves as 'awkward outcasts', claim to feel a connection with the killers - despite neither of the pair having been loners.

Columbine also created a 'common knowledge' playbook for school shootings, The Conversation reported, with dozens of perpetrators using the 1999 massacre as a model for their own attacks.

Robert Steinhäuser, who shot and killed 16 people at his secondary school in Erfurt, Germany, researched Columbine online before he carried out the deadly attack on April 26, 2002.

Eerily reminiscent of Harris and Klebold's attack, Steinhäuser dressed all in black and moved methodically through the school building, picking his victims one-by-one. He then turned the gun on himself.

Matti Saari, dubbed the 'YouTube gunman', shot and killed 10 students in an exam room at a college in Finland on September 23, 2008.

Saari, 22, carried out the attack 24 hours after posting a video on the internet in which he turned to the camera and declared: 'You will die next.'

Police ruled that Saari, who had been planning his rampage from as far back as 2002, drew inspiration from Columbine. He was reportedly fascinated with the killing and clips from Colorado massacre were listed among his favorite YouTube videos.

Investigators found a note in his home that said 'I have always wanted to murder as many people as possible'.

Nikolas Cruz, the 14-year-old who killed 17 people when he opened fire on Marjory Stoneman Douglas High School in Parkland, Florida, also researched the massacre before targeting his own high school. 

Investigators said that Cruz, who was sentenced to life imprisonment without the possibility of parole in 2022, aspired to be 'the youngest mass murderer'.

Similarly, Virginia Tech shooter Seung-Hui Cho claimed to be inspired by Harris and Klebold and called them 'martyrs' in his videotape suicide manifesto.

Adam Lanza, 20, who killed 26 people at Sandy Hook Elementary School had an 'obsession' with Columbine, police found. He idolized the killers and even created a Tumblr page that paid homage to the pair. 

Harris and Klebold also provided copycats with a depiction of how gunmen should dress, behave and speak. 

Santa Fe High School shooter Dimitrios Pagourtzis dressed in a long, dark trench coat - the same attire that Harris and Klebold wore during their rampage - when he shot 10 people dead on May 18, 2018.

Pagourtzis, 17, also shared a photograph on his now-defunct Facebook page of a military-style 'duster' featuring the same red star medallion worn by Klebold, according to The Houston Chronicle.

Wisconsin shooter Natalie Rupnow was seen wearing a KMFDM band t-shirt ahead of Monday morning's atrocity.

The German industrial band came into focus in the wake of Columbine. Harris had been pictured wearing the same branded black T-shirt before the massacre, and both he and Klebold were avid fans of the band.

KMFDM released a statement in 1999 expressing their sympathy in the wake of the Columbine shooting and distancing themselves from any Nazi associations.

Similarly, Vladislav Roslyakov - who launched a gun and bomb rampage at his college in Crimea, Russia in 2018 - is believed to been inspired by a music video dedicated to Columbine.

Roslyakov, 18, stalked his college halls with a shotgun and let off homemade explosives before killing himself in the library.

CCTV footage allegedly showed Roslyakov wearing a similar outfit to the portrayal of Harris in a music video for the anti-gun song Pumped Up Kicks by Foster The People, which was inspired by the Columbine massacre. 

It is suggested that Roslyakov copied the portrayal of Harris in the video by wearing a white T-shirt with black writing, black tight trousers with his hair combed back and a gun slung over his shoulders. 

The clip that Roslyakov is said to have copied was not from Foster The People's official music video. The band, whose bassist's cousin survived Columbine, said the song was inspired by school shootings. 

The Columbine effect goes far beyond online forums and internet searchers, with multiple shooters even saying they were inspired to carry out their own attacks after viewing mainstream media coverage about the massacre. 

Columbine was cited in court documents after a 15-year-old Oregon boy injured 10 students when he opened fire at Springwater Trail High School in 2007. He had recently watched a TV documentary about the killings.

Similarly, Caleb Sharpe, 15, had been watching documentaries about Columbine and the Sandy Hook massacre for about a year before he fatally shot a classmate and injured three others at his school in Washington state on September 13, 2017.

Court records revealed he watched the Mind of a Rampage Killer documentary at least 10 times before carrying out his attack, The Spokesman reported.

Another 15-year-old held his teacher and 24 classmates hostage at gunpoint in a five-hours standoff in 2010 after reading a book about Columbine. The Wisconsin teen shot himself as police stormed the classroom.

A Louisiana boy, also 15, in 2009 opened fire on his teacher after he ordered her to say 'Hail Marilyn Manson', which she refused. 

The teen, who kept a manifesto titled Deadly Diary II, was fascinated by Columbine and Manson, whose music was falsely linked to the Columbine killers despite the pair not appearing to be fans.

The Columbine effect also spawned concerning rhetoric surrounding school shootings. 

Some gunmen have reportedly discussed the massacre with their classmates, while others have joked about the incident.

Charles Andrew Williams, who shot two people dead and injured 13 others in California in 2001, even once commented that he was 'going to pull a Columbine'.

Perpetrators have also chosen the anniversary of the Columbine massacre as the date to carry out their own shootings. Threats of shootings are also regularly made on the Columbine anniversary date.