They have become one of the most notorious and alarming stripes of evil. People who, when you think back, seemed off. Didn’t dress right. Kept to themselves. Were nursing a bitterness that smoldered inside of them.

And then they picked up guns and went out and killed as many as they could.

In the aftermath, the same questions arise: Why didn’t everyone know? Why weren’t they stopped?

Now those questions are being asked about Christopher Harper-Mercer, who for reasons yet to be deciphered slaughtered nine people at Umpqua Community College in Roseburg, Ore., on Thursday. They have been asked about the man who killed nine people in a church in Charleston, S.C., in June. The man who killed six people in Isla Vista, Calif., last year. The man who killed a dozen people at the Washington Navy Yard in 2013.

And so forth.

What seems telling about the killers, however, is not how much they have in common but how much they look and seem like so many others who do not inflict harm.

Weaving a profile of the public mass murderer, drawing on threads that have been identified, can reveal the broad contours of a certain type of individual. But those contours are indistinct enough to apply to countless others — the recluse next door with poor hygiene who never speaks — who will never pick up a gun and go out and murder.

“The big problem is that the kind of pattern that describes them describes tens of thousands of Americans — even people who write awful things on Facebook or the Internet,” said James Alan Fox, a criminologist at Northeastern University who has studied and written about mass murderers. “We can’t round up all the people who scare us.”

The mass public killings that have drawn such intense public attention are a phenomenon that largely did not occur until two generations ago.

Image Christopher Harper-Mercer, 26. Credit... via Associated Press

Grant Duwe, a criminologist with the Minnesota Department of Corrections, has studied more than 1,300 mass murders that took place from 1900 to 2013. Of them, he classifies 160 as mass public shootings, ones in which at least four people were shot and killed in a concentrated period, excluding those in family settings or involving other crimes.

There were few before the 1960s. The episode, Dr. Duwe said, that some academics view as having “introduced the nation to the idea of mass murder in a public space” happened in 1966, when Charles Whitman climbed a tower at the University of Texas at Austin and killed 16 people.

Using data compiled by Dr. Duwe, the Congressional Research Service released a report this year that charted an increase in these shootings since then, from an average of one per year during the 1970s to four in the 2000s and a slight uptick in the last few years. The figures, however, are subject to intense debate, mainly over how to properly define the shootings.

Those who study these types of mass murderers have found that they are almost always male (all but two of the 160 cases isolated by Dr. Duwe). Most are single, separated or divorced. The majority are white. With the exception of student shooters at high schools or lower schools, they are usually older than the typical murderer, often in their 30s or 40s.

They vary in ideology. They generally have bought their guns legally. Many had evidence of mental illness, particularly those who carried out random mass killings. But others did not, and most people with mental illness are not violent.

“They’re depressed,” Dr. Fox said. “They’re not out of touch with reality. They don’t hear voices. They don’t think the people they’re shooting are gophers.”

‘History of Frustration’

They do not fit in. Their most comfortable companion is themselves. According to Dr. Fox, mass killers tend to be “people in social isolation with a lack of support systems to help them through hard times and give them a reality check.”