Six days before he allegedly opened fire on an elementary school playground, the eighth-grader returned to his Instagram group chat to fixate, yet again, on his most intense interests: guns and bombs and the mass murder of children.

"My plan," wrote Jesse Osborne, who had turned 14 three weeks earlier, "is shooting my dad getting his keys getting in his truck, driving to the elementary school 4 mins away, once there gear up, shoot out the bottom school class room windows, enter the building, shoot the first class which will be the 2d grade, grab teachers keys so I don't have to hassle to get through any doors".

He had been researching other school shooters for months and, determined to outdo them, learned exactly how many people they'd murdered: 13 at Columbine High; 26 at Sandy Hook Elementary; 32 at Virginia Tech.

Jesse Osborne, 15, is to be tried as an adult after killing his father and a 6-year-old boy in a school shooting.

"I think ill probably most likely kill around 50 or 60," Jesse declared. "If I get lucky maybe 150."

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On Valentine's Day, at the same time police say another angry teen, Nikolas Cruz, slaughtered 17 people at a Parkland, Florida, high school with a semiautomatic AR-15, Jesse was sitting in a South Carolina courtroom, waiting to find out if he would be tried as an adult for a 2016 rampage that left his father and a 6-year-old dead.

The two teens have much in common. Both, investigators say, tortured animals, obsessed over guns and bragged of their deadly intentions on social media. And in the hours after Cruz's alleged killing spree, as the nation began, once again, to ask why, a group of detectives, prosecutors and psychiatrists were providing answers about Jesse, now 15. He'd detailed his motives in dozens of online messages, in his 46-page confession and in lengthy interviews with doctors who evaluated him, offering extraordinary insight into the mind of an American school shooter.

For Peter Langman, one of the country's leading experts on the subject, the teen's calculated approach and lack of empathy called to mind Eric Harris, one of the Columbine killers Jesse idolised.

"The coldbloodedness, the callousness of the attack - not only before but afterwards," said Langman, who was not involved in the case but has reviewed Jesse's confession. "Even having done it, he's not struck with horror or guilt."

In fact, James Ballenger, a psychiatrist who interviewed Jesse for a total of nine hours, found that the teen revelled in what he'd done.

"He wants to talk about how dangerous he is," Ballenger testified. "He wanted people to know."

At the five-day hearing that began February 12, prosecutors pushed for Jesse to be tried as an adult because if he remained in the juvenile system, he could only be held until age 21. Jesse's defense team, meanwhile, tried to portray him as a lost but misunderstood child, alleging that he had been bullied by kids at school and mistreated by his father at home.

Jesse, who had grown up on his family's chicken farm, liked to shoot guns, but so did many boys his age in Townville, a rural community 40 miles southwest of Greenville. He camped with his grandparents, whom he adored, and watched the movie Frozen, one of his favourites. An avid reader of history, he told his family he wanted to fly to space when he grew up.

At odds with that portrait were Jesse's own words, captured in dozens of messages he'd exchanged in his private chat group, which the teen claimed included users from around the world.

"I HAVE TO BEAT ADAM LAZA . . .," he wrote nine days before the September 28, 2016, shooting in a misspelled reference to the Sandy Hook killer, Adam Lanza. "Atleast 40."

Two days later, he debated whether he should attack his middle school, from which he'd been expelled, or his elementary school, just up the road. He decided on Townville Elementary, because it was closer and had no armed security. Jesse, who considered himself the victim of an unfair world, announced online that he would kill kids he didn't know and had never met "before they bullie the nobodys."

"Itll be like shooting fish in a barrel," he wrote his friends, whose identities remain unclear, along with whether the FBI has tracked any of them down. The agency declined to comment, citing Jesse's open case.

In the chat, he said he had researched police response times for the area and found that it would take them 15 minutes to get there, and maybe 45 for SWAT. He said he would throw pipe bombs into each classroom before he got in a shootout with police and killed himself with his shotgun. He said he had been planning a massacre for two years.

A detective later discovered that Jesse, then a 182 centimetre-tall, 66 kilogram wispy-haired blond with a voice that tended to crack, had used his phone to Google these terms: "deadliest US mass shootings," "top 10 mass shooters," "youngest mass murderer," "10 youngest murderers in history."

Seven hours after he was pinned to the ground outside Townville Elementary by a volunteer firefighter, Jesse acknowledged in an interview with investigators that he'd shot far fewer kids than he'd intended. The problem, he explained, was the weapon. He'd only had access to the .40 calibre pistol his father kept in a dresser drawer. It had jammed on the playground, just 12 seconds after he first pulled the trigger.

The weapon Jesse really wanted, the one he'd tried desperately to get, was, the teenager believed, locked in his father's gun safe: the Ruger Mini-14, a semiautomatic rifle much like the gun that, 17 months later, was fired again and again at Marjory Stoneman Douglas High, during one of the deadliest school shootings in American history.

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His obsession began, he said, with a song that mentioned Columbine.

Jesse wanted to know what it meant, so he asked his father, who told him about the April 20, 1999, attack that became a seminal moment in the evolution of modern mass shootings.

"And so I researched more," he said during his confession. "Then I looked up all these other ones."

Through much of his childhood, Jesse had seemed no different from any other kid in the Southern community of 4000 people. Before he attacked Townville Elementary, Jesse had gone there through fifth grade, doing well in his classes and hardly ever getting into trouble. He played catcher on a rec-league baseball team. He got invited to birthday parties.

It wasn't until he moved to a middle school in a neighbouring county that his "other side," as one psychiatrist put it, became clear. He pulled the legs off crickets and smashed frogs against the ground and habitually watched a video of kittens being mutilated. He also posted Instagram videos about Columbine that some at the school considered a potential threat. The teen grew more volatile, insisting that he'd been bullied, a claim investigators later questioned.

After one kid poked his chest, Jesse threatened him.

"When I come back with a rifle, you're going to be the one I shoot," he recalled to Ballenger, who noted in court that Jesse "loved how much it scared the boy."

Then, one day, he brought a hatchet and a machete in his backpack. When another student spotted the weapons and reported him, Jesse was expelled and arrested, serving a brief stint in juvenile detention before being placed on probation.

It was then, as a home-schooler, that he became consumed with violent fantasies, the court evidence showed. How much his parents knew about them remains unclear, but at least once, the couple came across internet messages he'd written that they found disturbing, and his mother acknowledged to investigators that he'd become increasingly difficult to raise.

Jesse, whose older siblings had moved away, spent long stretches alone in his room, where he played first-person shooter games for hours and scoured the Internet for information on firearms. Beneath one YouTube video reviewing guns, The Washington Post found that he'd asked about the quality of a Hi-Point 9mm Carbine, an affordable semiautomatic rifle. In response to a question about gun licenses on another video, he wrote, "depends on your state. here in south caralina we dont need a lisense for anything except explosives or machine guns which are crazy expensive." At least once, he commented on a video titled "Active Shooter in the School - Plans and Drills."

Jesse told police that he had also discovered the "true crime community" on Tumblr, where fans of serial killers and mass murderers gather to delight in their shared devotion. Through that, his fascination with other school shooters, especially Eric Harris and Dylan Klebold, bloomed. His Instagram username included "nbk," for the movie "Natural Born Killers," and "kmfdm," for a German industrial band - a pair of pop-culture references that appeared frequently in the writings of the Columbine killers.

The pair's influence over the past two decades has been enormous, said Langman. He noted that Jesse was at least the 33rd gunmen to cite Harris and Klebold as an influence.

"Knowing about a school shooter doesn't cause someone to become a school shooter," Langman once wrote. "For people already at risk or on a path toward violence, however, external influences in the form of other mass attacks may be a factor in spurring them on toward committing their own attack."

That was clearly true for Jesse, who mentioned Lanza and Harris in his messages and Seung-Hui Cho, the Virginia Tech gunman, in his confession.

On the morning of the shooting, he preloaded a magazine for his dad's handgun, writing to his chat group that he would soon "be on the news." When his dad, who Jesse accused with little evidence of being an abusive drunk, "started fussing" at the boy, he retrieved the pistol, came down stairs and shot his father in the back of the head.

He said goodbye to his dogs and gave his favourite rabbit, Floppy, a kiss, he told investigators, before putting on a vest, packed with more than a dozen extra rounds, and heading to the school, where he arrived at 1.41pm.

"The first shot I hit someone. Second I hit a window," he told his interrogators.

"The first shot you hit someone?" one of them asked.

"I think. I just seen red pop up, so I just assumed."

And had his gun not jammed, they asked him, what would have happened?

"God knows," he said.

---

"Now I have a life," Jesse announced near the end of his confession. "Probably won't get a job, but I'll - I'll at least have a life."

That sounded bizarre for a teenager who knew he would likely face decades in prison, but, as the experts who would analyse him soon discovered, the comment spoke to Jesse's chief motivation.

Ballenger, a psychiatrist with 40 years of experience, had already analysed Dylann Roof, the white supremacist who killed nine African-Americans in a Charleston church in 2015, and Jared Lee Loughner, who killed six people and wounded Democratic Representative Gabrielle Giffords in Arizona in 2011. What he saw in Jesse was a young man who killed not because of bullies or abuse or a fractured mind, but because he wanted to attain the life and status he'd envisioned.

"He was going to be famous, the best shooter ever," Ballenger testified. "He was going to be worshiped for a long time - worshiped."

"Did you see evidence of him looking at statistics of people to see how he lines up?" a prosecutor asked him.

Ballenger noted Jesse's Google searches for other mass shootings before his attack.

"He actually confirmed that he would be one of the youngest, if not the youngest," the doctor said.

"And that was one of his goals?"

"That was his goal," Ballenger told the court. "To be the best shooter - to get 50 to 60."

But Jesse didn't kill that many, not even close, and in a country that barely notices school shootings in which just a single child dies, his brief assault garnered only momentary attention outside South Carolina.

For Townville, though, those 12 seconds of gunfire were devastating, leaving children, parents and teachers overcome with guilt and post-traumatic stress, as detailed in a Post story last year.

A teacher and a student survived their bullet wounds, but one round Jesse is accused of firing took the life of Jacob Hall, a bespectacled 6-year-old who was the smallest child in first grade. Hundreds attended his funeral, where his body laid inside a miniature gray casket topped with a Ninja Turtles figurine. He was dressed in a Batman costume.

Jesse showed almost no remorse, according to experts who analysed him. He mocked his victims and once suggested that he'd done Jacob "a favour."

In court, prosecutors presented evidence that Jesse's preoccupation with violence never receded. After a 15-year-old killed two students and injured at least a dozen other people at a Kentucky school on January 23, Jesse looked up the incident on a computer at the detention centre. He also repeatedly searched for the lyrics of a vicious rap song, "One Shot Kill."

"My observations of him in this courtroom over the last few days have been that he is still a hundred per cent in that mind-set," Ballenger testified, saying that he'd seen Jesse smile, over and over, as his crimes were discussed. "He's very comfortable in this hearing."

That's apparently not what Jesse had wanted to convey, though. After his arrest, he'd researched the symptoms of autism and schizophrenia, and in interviews with doctors, he talked of hallucinations that included seeing blue people and the cartoon character Scooby-Doo. His evaluators testified that, without question, he was lying.

Ballenger concluded that Jesse was at high risk for developing antisocial personality disorder, often called sociopathy, which cannot be diagnosed before age 18. He recommended that the teen be tried as an adult, and the judge agreed when he issued his February 16 ruling.

Never far from the courtroom discussion was the horror of what might have happened if the pistol he'd taken from his father's dresser drawer hadn't jammed or, even worse, he had used the semiautomatic rifle. Once, Jesse had pretended that he was texting on his phone as he tried to film his father typing in the code to the gun safe. Another time, he'd put dish soap on the buttons, hoping it would reveal fingerprints.

Jesse could have killed so many more children, he knew, with that Mini-14.

"To the media, it's called an assault rifle," he told interrogators before lamenting, for a second time, that it remained locked away.

But he was wrong.

Soon after the shooting, investigators searched the teen's home for evidence. In his parents' bedroom, they looked in the closet, and there, outside the safe and just feet from his father's dresser, was the weapon Jesse coveted.