A nightmare in northern California:

Four Northern California high school students have been arrested in connection with what law enforcement officials called a highly detailed plan to shoot and kill fellow students and teachers, just days after a mass shooting on a community college campus in rural Oregon.

The four, all students at Summerville High School in Tuolumne, Calif., “were going to come on campus and shoot and kill as many people as possible at the campus,” Sheriff James W. Mele of Tuolumne County said at a news conference on Saturday.

Sheriff Mele did not identify the students other than to say all were male juveniles. Tuolumne is about 120 miles east of San Francisco.

Investigators have found no clear motive for the planned attack, Sheriff Mele said, but he said that “the suspects’ plans were very detailed in nature and included names of would-be victims, locations and methods in which the plan was to be carried out.” The students planned to open fire during an event at school, he said, and “were in the process of trying to obtain these weapons.”

Officials said the plot was foiled on Wednesday when a group of students alerted a teacher after they overheard three of the four discussing a plan to open fire on the school.

If you read on in the story, the sheriff said the plot was far advanced; all they needed to do was to get their hands on weapons.

What if you discovered that you, or your child, was on the hit list? I don’t know that I could let my kid go back to that school again — even though the alleged would-be mass murderers were removed from the school, and perhaps were in jail. It’s so unnerving. It’s important to remember that these things are still exceedingly rare. Nearly every high school in this country carries on its business without having mass shootings, or breaking up massacre plots.

Still, the mystery of evil in these cases — that teenage boys aspire to do things like this, not on the spur of the moment, and not for any cause other than to cause death and suffering — is so unfathomable that one hardly knows where to begin. This, I think, is why we are so eager to identify simple causes and impose simple solutions. The radical irrationality of the acts is key to their terroristic nature. These terrorists don’t want anything in particular; they just want to kill.

What is it in contemporary culture that drives some young men to think like this, and to act on those thoughts? Where I live, there are so many guns around — it’s a hunting culture — that any kids who wanted to carry out this kind of atrocity could easily do so. But nobody ever does. And again, it’s important to remember that this is still, thank God, very, very rare. Even so, the fact that it happens at all is terrifying. If they were trying to bring about a revolution, or to do it to obey Allah, or anything like that, I could at least understand it. If they were maniacs acting on ungovernable impulse, that would be understandable, in its way. But this?

What is missing?