It is still virtually impossible, other than existing behavior, though studies continue to be done—I worked on one in the 1970’s—as this article from the Washington Post reports.

An excerpt.

“After every act of incomprehensible violence, the world asks whether the killer could have been identified ahead of time. It’s as automatic as the call for more gun control and better mental health services.

“Psychologists and psychiatrists have been working for decades to try to figure out whether there’s a link between mental illness and violence, and if so, which people are likely to act. Using an ever-changing tool kit of theories and questionnaires, they’ve made some progress.

“It’s now fairly clear, for example, that people with severe mental illness, such as schizophrenia, bipolar disorder and some personality disorders, are more likely to commit violent acts than others. But the risk is small. The vast majority of mentally ill people won’t commit assault, rape, arson or homicide, although the risk rises sharply among those who abuse drugs and alcohol.

“These insights are proving useful to psychiatrists, psychologists, judges, school administrators and others who must decide whether someone seems too dangerous to be left alone. But they aren’t good enough to identify an Adam Lanza, the young man who killed 28 people, including himself, in Newtown, Conn., last month. (Lanza’s mother told friends that he had Asperger syndrome, a developmental disorder, but no evidence has emerged that Lanza was diagnosed as mentally ill.)

“There is no instrument that is specifically useful or validated for identifying potential school shooters or mass murderers,” said Stephen D. Hart, a psychologist at Simon Fraser University in Vancouver who is the co-author of a widely used evaluation tool. “There are many things in life where we have an inadequate evidence base, and this is one of them.”

“Even when someone has a history of threatening behavior, the killing of innocent people can’t necessarily be prevented.

“The woman accused of pushing a man to his death in front of a New York subway train on Dec. 27 had been arrested several times for assault and treated in the psychiatric wards of two hospitals. The man who fatally shot two firefighters and himself in Webster, N.Y., on Christmas Eve had killed his 92-year-old grandmother three decades earlier.

“The task of identifying violence-prone individuals is even trickier with young people, who have shorter histories and whose normal development often includes a period of antisocial behavior.”