The historical evidence that terrorist attacks become blueprints for random massacres is slim, Dr. Gould and others said. No one knows precisely what factors prompt people to commit such extreme acts, when the primary motivation is radical ideology. In rare cases where perpetrators survive, even they often do not have a clear sense of what moved them from despair and anger to large-scale murder.

“In interviews, they come across as what we call pseudo-terrorists,” said J. Kevin Cameron, the director of the Canadian Center for Threat Assessment and Trauma Response, who has consulted on school shootings and other mass killing for almost 20 years. “They’re people with some ax to grind who are fluid — that is, they’re truly at their core struggling with suicide and homicide, and they swing between the two. Today the person is more suicidal; a week later he’s more homicidal.”

But there is reason to suspect that contagion is a factor, from previous research on violence. Researchers have long known that highly publicized suicides can precede “clusters” of suicides in the weeks or months afterward, in people already thinking about suicide. The likelihood of such contagion depends on the prominence of the coverage, the detail in the reports about methods, the richness of the portrayals of people affected. In similar fashion, terrorist attacks and mass killings have been exhaustively covered, Dr. Gould said.

The vast majority of people who take their lives kill only themselves, leaving no evidence that they wanted to kill others. But experts suspect that murder-suicides are subject to contagion effects from high-profile cases, though the numbers are too small to establish that statistically. Only about 1 to 2 percent of murder-suicides target random people outside immediate family or friends, said Matthew Nock, a psychologist at Harvard.