I RETURNED to my office in Hartford late one afternoon to find a future mass murderer sitting in my chair.

The visitor, Matthew Beck, was not yet a killer. He was a mentally ill accountant from the Connecticut Lottery telling tales of corruption at his office. He had pulled up a seat — mine — at the newspaper where I worked for an interview with one of my colleagues, who expressed relief afterward that the intense Mr. Beck did not have a gun.

Mr. Beck, however, did have guns. And a few months later, he brought one to work and fatally shot four top lottery officials before killing himself. The mass shooting, in March 1998, was the second worst in Connecticut until three years ago, when Adam Lanza killed 20 children and six adults at Sandy Hook Elementary School in Newtown, where my wife had once applied for a job.

I thought of both of these incidents in recent weeks as horrific shootings shattered communities in Charleston, S.C., and Chattanooga, Tenn. On Thursday, there was yet another shooting, this one in a Lafayette, La., movie theater by a gunman with a history of mental instability who left a trail of angry Internet postings. A familiar, achingly unsatisfying search was underway for answers to an old question: What causes someone to take innocent lives? The usual suspects were lined up: mental illness, twisted ideologies, substance abuse, a culture awash in guns.

This vexing issue was what my fellow Times journalist Michael Luo and I spent a year examining following the Sandy Hook massacre. After reviewing more than 1,000 court cases and forensic reports, conducting countless interviews and producing seven stories, we were certain of one thing: It was far too easy for firearms to fall into the wrong hands.

But the motivations behind the actions of those hands often remain maddeningly opaque. How did Dylann Roof, a 21-year-old high school dropout who drank and did drugs, come to immerse himself in white supremacism and end up accused of gunning down nine African-Americans at a Charleston church? In Chattanooga, why did Mohammod Abdulazeez, 24, a college graduate struggling with depression and drug abuse, gravitate to radical Islam and fatally shoot five servicemen?

Mark Potok, who has spent decades researching hate groups and their followers for the Southern Poverty Law Center, said many shooters turned out to be “people who are looking for something larger than their own small lives, to be seen as a hero standing up for a cause.” Suspects like Mr. Roof go on the Internet and discover groups that validate their feelings and offer a sense of belonging, he said.

“It was once viewed as very unlikely that a person could be radicalized solely through a computer screen, but something has changed,” Mr. Potok said. “Today, the Internet really is the language of young people like Dylann Roof.”

As for Mr. Abdulazeez, he appears to have researched Islamic martyrdom on the web, although it was not clear he ever directly communicated with anyone espousing terrorism. The specter of self-radicalized “lone wolf” killers in the age of the Internet has added a frightening dimension to the 21st century variant of the mass shooter.

The widespread availability and glorification of firearms also cannot be overlooked as an important ingredient in this toxic mix, said Brad Bushman, a psychology professor at Ohio State University who served on a White House task force on gun violence after the Sandy Hook shooting. Numerous studies have shown that the mere presence of a weapon can make people more aggressive.

“I think guns are great equalizers, especially for people with limited voice, people on the fringes of society who may be rejected by peers or are trying to make themselves feel more accepted,” Mr. Bushman said. “Guns make people feel more powerful.”

For all their differences in motives and targets, mass shooters fall along a continuum of violence that is unnerving in its steady forward march. An F.B.I study found that between 2007 and 2013, there were an average of 16.4 such shootings a year, compared with 6.4 from 2000 to 2006. It is hard not to feel a growing sense of foreboding and futility, that we cannot escape their recurrence. Each new case delivers not just a blow to our collective conscience, but a there-but-for-the-grace-of-God moment.

One result of the Connecticut Lottery killings was a state law enabling the police to more easily seize firearms from people deemed to be a threat. The court records of these cases form a catalog of near misses and hint at the pervasiveness of firearms among all social strata.

There was the 24-year-old man found sitting in his car outside an ex-girlfriend’s place, crying and holding a gun on his lap. After disarming the man and getting him to a hospital, police officers entered his home and discovered seven high-powered rifles, a gas mask, knives and a backpack filled with 1,000 rounds of ammunition and survival gear.

A 27-year-old insurance company employee with bipolar disorder was found walking around with a gun in his pocket and six more stashed in his house, after a concerned relative reported that he was behaving strangely. Then there was the paranoid schizophrenic, 55, who had 18 guns taken away after he threatened to kill his mother and a nurse. When I interviewed him at his trailer near some railroad tracks in 2013, he told me how important his firearms were to him, showed me his empty gun safe and said he intended to refill it.

Indeed, one is struck by the nexus of mental instability, substance abuse and easy access to firearms. A regulatory loophole and bureaucratic bungling allowed Mr. Roof to buy a handgun despite a drug arrest that should have prevented it. Mr. Lanza, a deeply disturbed loner fascinated by mass killings, lived with his mother, who legally owned a small arsenal. And Mr. Beck was able to keep a pistol permit, even though he had been hospitalized twice for depression.

But, of course, most people with mental health problems are not violent, let alone prone to mass murder. What makes someone seek solace in a spasm of bloodshed is perhaps unknowable. That sense of mystery was captured in a letter released publicly by Mr. Beck’s parents after his killing rampage. In it, Donald and Priscilla Beck lamented that despite “the love and help from friends and family and the treatment, counseling and medications from his doctors, he chose the wrong path.”

“We love you Matt,” they wrote. “But why?”