Many shooters do not have a history of violent crime that might help predict such an incident. Still, a heightened awareness may be the ripest area for preventing mass shootings: Small but troubling hints could be an indication that a bigger problem is brewing.

Sometimes, though, these warning signs go completely unnoticed. According to news reports, Holmes mailed a University of Colorado psychiatrist a journal with details and drawings of how he was going to commit a massacre. The package arrived July 12 but hadn’t been opened before the shooting, according to one report. The university says it got the package the Monday after the shootings.

4. Communities come together after mass shootings.

They do but often only temporarily. I reported from the Columbine and Aurora shootings from the first hours and through the first days (and 13 years and counting in the case of Columbine). I also traveled to Virginia Tech in 2007 days after the shooting on that campus.

More than $4.6 million in donations was raised for Columbine victims, but distributing it brought gut-wrenching questions and debates over how much to give to the families of those who died versus the injured.

Lawsuits, which usually follow mass shootings, also foster acrimony within the community. According to a study of the 1997 West Paducah, Ky., school shooting, suits were filed against students who had heard that something was going to happen and even against producers of the movie The Basketball Diaries, which the shooter reportedly watched. The victims’ families who brought the suits said acquaintances shunned them, they received hate mail and they were accused of wanting only money and stalling the healing process.

And soon after the Aurora shootings, another hyperpartisan debate on gun control began.

There is a sense of unity after mass shootings. But the threads begin to quickly unravel.

5. It can happen anywhere.

Yes, but mass shootings at schools tend to occur in suburbs and small towns, where high school is the main driver of social status. Students who feel like outsiders have few, if any, other places to turn for friends and self-esteem.

School shootings also tend to occur in the South and the West, where researchers have identified a “culture of honor,” in which people place a high value on their reputation and, in some cases, are willing to fiercely defend it to the point of violence. This culture comes from long-standing regional traditions that combine chivalry with the need to defend one’s property in places where law enforcement was sparse.

It has been translated to the schoolyard by shooters who retaliate with violence when they feel they have lost their status. A 2009 study by researchers at the University of Oklahoma found that states with a culture of honor had more than twice as many school shootings per capita as other states. Mapping just some locales shows the pattern: Bethel, Alaska; Springfield, Ore.; Littleton, Colo.; Pearl, Miss.; and West Paducah.

Similar issues of vengeance for a wrong, or a perceived wrong, committed by individuals or society are also typical with adult shooters.

Jeff Kass, a former reporter for The Rocky Mountain News, is the author of “Columbine: A True Crime Story.”