Slowly, amid rumor and misinformation, a picture of the killer is emerging, and it is dismayingly familiar. Adam Lanza was yet another young, withdrawn, middle-class male who for some unimaginable reason graduated from his adolescence as a mass murderer.

What happened Friday in Newtown, Conn., was a variant on what the country has witnessed repeatedly in recent years. Once again, it was a pseudo-commando attack, as if the killer were playing a video game and racking up points for every victim. Once again, the crime appeared to be staged for maximum shock value. And once again — just as in Aurora, Colo., this past summer — there was the element of overkill, with multiple weapons, a military-style rifle and massive amounts of ammunition.

Lanza, 20, shot his mother, Nancy Lanza, at home, and then invaded Sandy Hook Elementary School, where he killed 20 first-graders and six adults, firing 30-round clips from a .223 Bushmaster semiautomatic rifle before taking his own life. How many bullets? “Hundreds,” Connecticut State Police Lt. J. Paul Vance said Sunday.

Detectives recovered evidence at the Lanza home that might help explain the killer’s thinking, but authorities have not revealed what they found. For the moment, the Newtown massacre remains as inexplicable as it is horrifying. “We don’t have a specific reason,” Vance said.

The location and lethality of the Newtown tragedy, and the preciousness of the victims, have turned the holiday season into a period of national soul-searching and calls for action to curb gun violence. Newtown dominated the Sunday talk shows. President Obama flew to Connecticut to join the mourning, and people nationwide revisited with new urgency the complex issues of gun laws, mental illness and access to mental health care.

The Newtown discussion necessarily sweeps in the news media, which give the killers a notoriety they couldn’t have achieved legitimately. The discussion touches on Hollywood, which markets spectacular make-believe violence. Also implicated: The computer gaming industry, which profits from ultra-realistic shooting games that are bloodier than ever.

“I point the finger unreservedly at the entertainment industry, which has spawned and cultivated gaming that by design is increasingly real, geared to action as the shooter’s point of view, increasingly dehumanizes victims, and increasingly rewards players by how many they kill,” said Michael Welner, a forensic psychiatrist and chairman of the Forensic Panel, who works on more than 20 homicide cases a year.

Although the profile of the mass shooter is often a familiar one, that is true only after the fact — when it’s too late. And the overwhelming majority of young men who play violent video games and are social misfits do not commit any crimes at all, much less shoot up a grade school.

How does society tell the truly dangerous ones from the ones who are just a little weird?

“We’re not even good at predicting minor violence. When you’re talking about preventing a mass shooting, that’s a needle in a haystack,” said Jeffrey Swanson, a professor in psychiatry and behavioral sciences at Duke University School of Medicine. “You can’t just go out and lock up all the socially awkward young men in the world.”

Most gun violence, he said, involves suicide, not homicide. And he noted that access to highly lethal weapons makes a difference, citing the school attack Friday in China in which a knife-wielding man wounded more than 20 students: “The point is, they were injured” — not killed — “because the lethality of a knife is so much less than that of a gun.”

It remains unclear why Lanza targeted the school. Connecticut Gov. Dan Malloy said Sunday that Lanza had attended the Sandy Hook Elementary School years ago. But one classmate at a middle school told The Washington Post that Lanza had been home-schooled until seventh grade.

So far there is no credible evidence that Lanza tried to document his crime or boast of his plans, as some mass murderers have. Lanza had no social media footprint — not even a Facebook page.

Another bizarre detail that has not been officially established and may or may not be significant: Lanza couldn’t feel physical pain, according to an Associated Press report.

‘‘If that boy would’ve burned himself, he would not have known it or felt it physically,” Richard Novia, who knew Lanza when Novia advised the high school’s technology club, told a reporter for the Associated Press.

Former classmates from high school recall Lanza as a quiet, geeky kid who carried a briefcase instead of wearing a backpack. There is no evidence that Lanza was bullied at school. His parents were divorced, and he lived with his mother. There is no evidence that Lanza suffered from any abuse in the home.

Some news reports have suggested that he had a mild form of autism. But that has not been officially confirmed, and moreover, the link between autism and violent behavior is weak or nonexistent, according to numerous studies.

Schizophrenia, however, appears to increase the risk of violence, particularly when the illness is combined with substance abuse.

Some experts believe that impairments in three mental functions may sometimes lead to violence when they occur together. The three are the ability to imagine oneself in someone else’s situation (called “theory of mind”); the ability to damp emotions (“emotional regulation”); and the ability to think about what one ought to do (“moral reasoning”).

Experts, however, caution against drawing inferences from sketchy information.

“The one thing that we must not do is look for a quick fix for our grief by in any way assuming that the diagnosis is equal to the cause of this violence,” said Harold J. Bursztajn, a psychiatrist and co-founder of the Program in Psychiatry and the Law at Harvard Medical School.

Newtown may forever remain fundamentally inexplicable. The perpetrator is dead. So is his mother. He appears to have had no close friends. His father and brother lived far away and do not seem to have been close to him.

Coroner Wayne Carver finished the autopsies on Lanza and his mother Sunday. As of late afternoon, he said, no one had come to claim the bodies.

Peter Hermann in Newtown and David Brown in Washington contributed to this report.