The 15-year-old had repeatedly threatened to bring a gun to his school in the San Diego suburbs of Santee, southern California, but no one took him seriously enough to report the threats.
Then, on Monday, smiling as he fired, Charles Andrew Williams emptied his handgun three times into the ranks of his fleeing schoolmates. He fired more than 30 shots, killing two and injuring 13.
Police said yesterday he was driven by "rage", and shocked classmates say that the slight young man, described by many as "normal", had been picked on and bullied. Williams is due to be charged today as an adult with the killings under new California legislation, approved by referendum last year, which makes it easier for teenagers to face adult charges. If convicted he is likely to face a life term.
The attack confirms in a grisly way the pattern observed in a major 1999 Secret Service report on such killings, which found that more than three-quarters of those involved in some 37 incidents surveyed had told at least one other person of their intentions in advance.
Some schools have since developed early-warning programmes and there have been at least four successful tip-offs in the last month which have prevented similar attacks.
Williams is understood to have jokingly told up to 20 separate friends of his intentions and some had believed him to the point of frisking him on Monday. Unfortunately, he was carrying the gun in his backpack and they missed it.
Last weekend he had spent the night at the house of a friend, Josh Stevens. Stevens's mother's boyfriend, who was in the home that night, told the Los Angeles Times that he had heard the boy threaten to go on a shooting rampage.
But Mr Chris Reynolds said later that he could not be sure the boy was serious. He said he warned the youth that he would call sheriff's deputies if he got any inkling that he really intended to harm anyone. "I should've stepped up even if it wasn't true . . . to take that precaution," Mr Reynolds (29) said. "That's going to be haunting me for a long time. It just hurts because I could've maybe done something about it."
Williams had said he would use his father's guns, which he said were kept in a locked case, a friend, Dustin Hopkins (15), said. But when pressed about what he would do, Williams backed down, insisting he was joking and that he didn't even have a key to get to the weapons.
Police say he managed to procure the key.
The killings are likely to re-ignite arguments about gun control. California Democratic Senator Diane Feinstein said it should be a wake-up call. In particular, she said, mandatory gun locks could make it more difficult for precisely such cases.
While students are more likely to see guns in inner-city schools, the Santee shootings confirm that such killing sprees by disturbed individuals, invariably boys, usually white, tend to defy profiles.
Mayor Randy Voepel insisted on Monday afternoon that anybody looking to find a sick community would not find it in Santee.
