A teenage boy who was planning a Columbine-style massacre at his Yorkshire school confessed to a friend, teacher and police officers but was left free to continue the plot, it has emerged.

Prosecutors said officers did “not respond adequately” when the alarm was raised about bloody plans being drawn up by Thomas Wyllie and Alex Bolland, who were just 14 at the time.

The pair, who have now been jailed for a combined total of 22 years, started discussing a school shooting and reading bomb-making instructions in May last year.

​Bolland wrote that he “really wanna shoot up the school” in Northallerton, as Wyllie stockpiled explosive components at a hideout and tried to gain access to guns at his girlfriend’s house.

In September, Bolland told a schoolgirl that they were planning to carry out a shooting via Snapchat.

When she asked if he was joking, he responded: “No. No one innocent will die. We promise.”

The following day, the girl showed his disturbing messages to teachers who questioned the teenagers.

While Wyllie denied the plan existed, his younger classmate made what prosecutors described as “clear and unvarnished confessions”.

Snapchat messages from the second defendant to a friend in September 2017 (North East CTU) (North East Counter Terrorism Unit)

A teacher who spoke to Bolland told Leeds Crown Court: “He started off by saying that he had sent the messages because some students were making his life in school intolerable.

“He felt that they needed dealing with severely. He said they needed killing.

“He said he needed to eliminate these people, who were infecting the gene pool.”

Describing the conversation as “the most dreadful thing that a student has ever said to me”, the teacher added: “He was emotionless about the plan. He seemed to feel that it was something that needed to be done.”

The same teacher was informed by a police officer that his name was one of several on a “hit-list” of targets drawn up by the teenagers.

Police spoke to both plotters separately at home after the incident, when Bolland told them he “planned to go into school with a firearm in order to get rid of those who had wronged them” but Wyllie denied everything.

Prosecutors said North Yorkshire Police did “not respond adequately” to the threat until a specialist counter-terror team took over the investigation a month later.

Speaking after the teenagers were convicted of conspiracy to murder, North Yorkshire Police said the case had been reviewed and issues were addressed with several officers and staff.

A chat between the two boys discussing the plot (North East Counter Terrorism Unit)

Assistant chief constable Phil Cain added: “We fully accept that standards of investigation and our initial responses, to some of the incidents, did not meet those standards that are expected of us and what we strive to deliver.

“These were unusual circumstances, and once the seriousness of the allegations were realised, we turned to the North East Counter Terrorism officers for their specialist assistance and support.”

Counter-terror police arrested the pair in October, when Bolland changed his story to claim that he believed his co-conspirator was “joking”, before saying that he had felt “scared and isolated” as the plan started to get more serious.

In his own police interview, which took place at a similar time, Wyllie admitted that he had a “fascination” with the “morbid psychology” of prolific killers.

Discussing a diary in which he outlined these feelings, he told officers: “It was an admiration of these killer-type people – you know, when people die and you find their diaries.”

When asked about the detailed nature of the plans he had outlined in the journal, he said: “It was just getting stuff out of my head, and they are basically a sort of therapy.”

Leeds Crown Court heard that Wyllie “idolised” Eric Harris, who massacred 13 victims with accomplice Dylan Klebold at Columbine High School in Colorado before turning a gun on himself.

The 1999 atrocity was followed by numerous mass shootings committed by school pupils in the US, as well as plots directly inspired by Columbine.

A page from one of the plotters’ diaries (North East Counter Terrorism Unit)

Justice Cheema-Grubb, who jailed Wyllie for 12 years and Bolland for 10, said their plan “was not wishful thinking or fantasy, it was a real plot”.

“You are both 15 years old and you were only 14 last year when you planned to murder teachers and pupils at your school in North Yorkshire by shooting them in a re-enactment of the Columbine massacre,” she added.

“It was a firm plan with specific targets in mind as well as a plan to make indiscriminate explosives.”

Justice Cheema-Grubb said the boys had intended to cause “terror on the scale of the school shootings that have been seen in America”.

Police uncovered a diary where Wyllie espoused “far-right wing ideology” and discussed his motivations for wanting to carry out an attack.

The first page read: “If this is found I have committed one of the worst atrocities in British history or I killed myself.”

“F***, I hate my school. I will obliterate it. I will kill everyone,” he wrote in October last year. “I have a plan, a great f***ing plan.”

The diary said Wyllie would “lay low” in Catterick before murdering his ex-girlfriend’s parents and stealing her father’s guns, the court heard.

“I’ll make some explosives then well [sic] find a way back to Northallerton and well begin our assault on that f****** school,” it continued. “I just want to kill every single one of you f***ers.

“Everyone is filthy and deserve to be shot, including me. I’ll play the role of god and decide who a (sic) let live and die. Humans are a vile species which needs to die out.”

A rucksack found at an abandoned shack used by boys planning a shooting massacre at their school contained a mask, nails and a flammable liquid (North East Counter Terrorism Unit)

Officers later searching his hideout discovered a rucksack filled with screws, boards, and a flammable liquid that prosecutors said were components for an explosive device.

Wyllie’s girlfriend, who started dating him in June 2017, claimed he described her as “his Dylan Klebold” and encouraged her to give him access to her father’s shotguns.

He had carved his name into his girlfriend’s back and after her parents banned them from seeing each other, he threatened the family with a knife.

Detective chief superintendent Martin Snowden, head of Counter Terrorism Policing North East, said: “Whatever their motivation, the intent of the defendants and the direction of their actions, placed others at risk.

“Thankfully, we’ll never know if they’d have followed through with their plan.”

Mr Snowden warned that young people were “vulnerable to external influences, both in the real world and online”.

“While these influences are very difficult to control, it’s important we’re alive to the display of attitudes or behaviour which concern us and have the confidence to report them,“ he added.