To the rising pile of shooting rampages, Americans can now add the rapid-fire murder of 12 people at the Washington Navy Yard. It is a sign of our remarkable times that this horrid deed seems to pale next to the massacre of 20 schoolchildren in suburban Connecticut last December.

Behind virtually every one of these slaughters is a loner who had shown signs of being mentally ill. The Navy Yard suspect, Aaron Alexis, had complained to police in Rhode Island of enemies passing vibrations through hotel walls. He was questioned in Fort Worth, Texas, for firing a bullet into an apartment ceiling and in Seattle for shooting out a car’s tires.

Though every incident pointed to a sick mind, none was serious enough to raise a flashing red flag. Worrisome how many unbalanced people fly below the official radar.

It’s hard to believe there are more mentally unwell people in America than elsewhere. But there are more of other troubling things in this country: isolation, a mesmerizing parade of violent images and easy access to weaponry.

I’m not going to dwell here on the gun control issue except to say this: It’s one thing to want firearms for hunting or self-defense. It’s another to demand a right to own weapons that can murder large numbers in seconds. That reflects a cracked worship of killing power, especially attractive to the unstable.

Many argue that mental illness, not the flow of guns, drives these crimes. They are not entirely wrong. But how do you keep killing machines out of crazy hands? Laws requiring a sanity check for gun buyers sound sensible, but the guns used by the slayer of the schoolchildren in Newtown, Conn., were bought by his supposedly rational mother. Adam Lanza’s mother went to bars to brag about her guns while leaving them unlocked at the home she shared with her clearly troubled son.

We learn that Alexis, like Lanza, like the Columbine High School shooters, spent long hours hypnotized by violent video games. So pervasive have these games become that the public now shrugs at the likes of “Grand Theft Auto,” once considered shocking for its anti-social violence. The casual bloodletting in the new “Grand Theft Auto V” is said to be oiled by humor and satire, injecting more confusion into already-confused minds.

There is debate on whether these games promote violent behavior. The case that they do seems strong enough to have compelled one video game maker to hire a crack lobbying firm to stop a Senate bill that would sponsor research into the possible connection.

Much research suggests that ordinary people playing violent video games do experience heightened feelings of belligerence, along with higher heart rate and blood pressure. In his own study, Brad Bushman, a professor of communications and psychology at Ohio State University, found that typical college students playing violent games for only 20 minutes a day for three days became more aggressive.

Most players don’t act on their anger, because they come to the game in fairly good mental health, Bushman wrote in response to the Navy Yard massacre. “But what about players who already are predisposed to violence?” He added, “Violent video games are just one more factor that may be pushing them toward violence.”

America’s mass shootings seem to be about several things. They’re about a culture that bombards people with images of casual homicide, that likes to wave guns, that doesn’t pay enough attention to mental illness. Though mass killings occur in other developed countries, our especially deadly mix of factors may explain why they happen here with grotesque predictability. It’s something toxic in the air.

Froma Harrop is a syndicated columnist. Follow her on Twitter @fromaharrop.