When a student in England gathered bomb-making materials last year and drew up plans to attack his school in revenge for bullying, he was brought to the police's attention thanks to a US teenager in an internet chatroom.
Now the man given the task of preventing school shootings in the wake of the Columbine attacks in 1999 has revealed how this remarkable transatlantic arrest resulted from a decade-long strategy to counter school violence.
Robert Fein, a former director of the US National Violence Prevention and Study Center, who has 35 years of experience studying targeted violence, said research after Columbine showed that almost all the people who planned attacks on schools had confided their intentions to another student.
"What we know is that in roughly 80 per cent of the shootings we studied where kids attacked, other kids either knew about the planned attack or knew that a given student was looking to do something bad at school, but they didn't come forward," Dr Fein told TES.
That insight resulted in the Safe School Initiative in 2002, which was designed to advise schools on how to ensure that students felt confident to raise their concerns about their peers and to trust adults to deal with them.
Dr Fein said it was often difficult in the wake of a tragedy to establish why students had not raised the alarm but he identified some common themes.
"They say, we didn't believe he was really going to do it ... he smiled when he said it. Sometimes they said they didn't know who to tell," he told TES, ahead of next week's UK television premiere of a PBS documentary on preventing school attacks.
The size and anonymity of US schools has also played a part. A belief that adults had given up on them was a common factor in many of the 37 school shootings, involving 41 perpetrators, that the Safe School Initiative studied. "We didn't find one young person who carried out an attack who had a real relationship with a trusted adult," Dr Fein said.
Many schools have now instituted plans to ensure that every student has one adult who is directly responsible for their welfare and pastoral care, he added.
This cultural change paid off in unexpected fashion when in February last year a US teenager joined the internet chatroom Omegle, which pairs users at random for one-to-one discussions. His partner, identified in the website's style only as "stranger", made wild and frightening claims.
According to prosecutors, he said "20 minutes from now he would be armed with a Magnum.44 revolver, a Beretta 92FS and various other weapons and ammunition. He said he would keep shooting until the police arrived, at which point he would commit suicide."
The teenager called the FBI, who alerted British police. Eventually they traced the comments to a 16-year-old boy with Asperger's syndrome in Northamptonshire, in the Midlands of England, who confided his feelings of isolation and victimisation in a journal, while making plans of classrooms detailing whom he would target. The police also found instructions on bomb-making and two of the chemicals used in the making of gunpowder. He was sentenced earlier this year to indefinite custody in a secure mental unit.
But even now, it is far from clear that he really intended to carry out an attack. His psychiatrist, Dr John O'Brien, told the court: "He has talked about this in a fantasy-type way, and has said he never planned for it to be real life and he didn't have plans to carry it out. He knew it was a crime and was wrong."
About 120 school attacks are believed to have been prevented under the Safe School Initiative. Dr Fein conceded that many of those might never have come to fruition. But he said the conspirators were often glad to have been stopped by authorities so that they could receive help.
"We don't know how many people in the past might have planned something and never carried it out. We don't know how many people might have given up on their own or might have been stopped without it coming to public attention,"he said.
"I know of one circumstance when a young man was stopped from making an attack and was brought into the criminal justice system. Several years later he wrote to thank the person who had brought his plans to the attention of authorities - for saving his life and the lives of other people."
One of the school shooters interviewed for the research said that his obsession with his plan "was like an avalanche, I was totally focused on it" and that "I wish someone had turned me in", according to Dr Fein.
For schools, he said the priority was to build a culture of respect, safety and trust, where students feel free to voice their concerns about classmates, and where feelings of isolation and grievance do not build up.
"Why the school?" Dr Fein said. "They said that's where the source of their pain was."
Path to Violence will be broadcast in the UK on Wednesday 22 May at 9pm as part of PBS America's Guns in America season (Sky channel 166 and Virgin Media 243).