It takes neither a clairvoyant nor a morbid personality to sense that, in time, the nightmare will return. We have already seen it often enough: Someone goes berserk and shoots up a school — typically a young man who turns a gun on himself after taking children’s lives, shattering families and leaving a tormented country to ponder how things went terribly wrong. Once again.

School shootings have become so much a part of the American experience that many people assume the atrocities are happening ever more frequently. But are they? The question is not simple, and evidence can be murky, if only because studies using differing methodologies arrive at varying conclusions. In search of answers, Retro Report, a series of video documentaries exploring major news stories of the past and their lasting impact, examines one of the worst shootings, a moment in 1999 so searing that it is still instantly evoked with a single word: Columbine.

The massacre of 12 students and a teacher at Columbine High School, outside Denver, was not the first mass killing at a school. By then, dots on the map like Jonesboro, Ark., and West Paducah, Ky., had been pushed into the national spotlight because of such shootings.

Retro Report Voices: The Student

Columbine, however, was different. Not only was it the most lethal attack on an American high school, but it also unfolded on television in real time, with teenagers cowering under desks and using mobile phones to report what they had seen or heard. It was “the first major hostage standoff of the cellphone age,” Dave Cullen wrote in “Columbine,” his well-received 2009 book about the ordeal. The police, he said, “had never seen anything like it.”

Columbine became a model for subsequent shootings. In writings they left behind, the young men who killed dozens at Virginia Tech in 2007 and in Newtown, Conn., in 2012 spoke of being inspired by Columbine. Yet as Mr. Cullen makes clear, and as the Retro Report video demonstrates, much of what the public came to believe about Columbine was flat-out wrong. Myths took root from the start, nurtured by frightened and confused students and amplified by news outlets running hard with rumor and conjecture.

The story line that unspooled ran like this: The student killers, Eric Harris and Dylan Klebold, belonged to a group at the school called the Trench Coat Mafia, said to be goth outcasts. The pair had been subjected repeatedly to bullying, especially by self-impressed jocks. And so this was a mission of revenge. The two young men went hunting for athletes, nonwhites and those professing a love of God, singling them out as targets before holing up in the school library and turning their guns on themselves.

Almost none of this proved to be true. In fact, as Mr. Cullen notes, Columbine was less a successful school shooting than it was a failed school bombing. Harris, later deemed a psychopath by some psychologists, and Klebold, a suicidal depressive, did not seek out specific groups or individuals. They wanted to blow up the entire place and kill hundreds indiscriminately with a homemade propane bomb. But their device failed to detonate. Only then did they begin using the guns that had unlawfully been supplied to them by others.

Yet the “loner versus bullies” vengeance template came to be widely accepted and applied to later shootings. It can be the sort of facile notion that appeals to those eager for a rational theory to explain an action that is utterly irrational. It can also be unnerving. What high school does not have its share of student bullies and oddballs? Not surprisingly, some parents live in fear that their own children could become tomorrow’s victims.

How severe is the threat of school shootings? Certitudes are elusive. Some analyses show an increase over the years, others a decline. One complication is a dearth of research drawing on universally accepted data. Some studies include stabbings in the tally of shootings, or events that occur not on school grounds but nearby, or suicides, or accidental discharges of weapons, or gang fights bearing no resemblance to the Columbine-style massacres that people tend to think of when they hear of school shootings.

Other researchers omit some or all of those categories in chronicling the mayhem. Also, many studies rely heavily on newspaper and television reports, a methodology that may be inherently flawed, given that some situations may well have been overblown by the news media and others ignored.

Retro Report relies on numbers compiled by the Centers for Disease Control and Prevention, based on known school homicides. That standard, too, may be an imperfect guide, but it deals in something tangible that is hard to gainsay: a body count. C.D.C. statistics suggest that shooting homicides in schools have held fairly steady across the last two decades, ranging for the most part between 15 and 30 a year. With well over 100,000 primary and secondary schools in the United States, the odds of any one of them coming under assault seem minuscule.

Some scholars recommend examining school shootings not as a discrete phenomenon but, rather, as one aspect of this country’s broader wave of gun violence. One such expert is Dewey G. Cornell, an education professor and forensic clinical psychologist at the University of Virginia. “We have a flood of gun violence in the United States,” he told Retro Report, citing an average of more than 300 shootings and 80 deaths a day. “Those occur throughout our community, not just in schools.”

For sure, schools must be kept safe, he added, but “we need to think about where that flood is coming from, and address the risk factors and causes of gun violence, rather than focus specifically on the schools.”

Risk factors plainly include the easy availability of guns, for the public in general and for the mentally troubled in particular. A debate is also underway over how extensively news organizations should dwell on the details of a mass shooting. That issue has been raised anew with the mass killings this year at a black church in Charleston, S.C., and the fatal shooting of two television journalists during a broadcast in Roanoke, Va. Some propose not even naming the killers, depriving them of the notoriety they seek while perhaps eliminating a possible source of inspiration for would-be copycats.

Then again, all venues are not equal.

New Yorkers, for instance, react more powerfully to a killing in Central Park, their secular cathedral, than to one that takes place just a block or two away; dread and a sense of personal vulnerability touch millions of people. Similarly, terror in a classroom sends shock waves rippling far and wide, whereas slayings at other locations often do not. Even a single school shooting is too much for many people to bear. After all, these are our children.