WASHINGTON ― Did our angry political culture help motivate Jared Lee Loughner on what authorities say was his mad shooting spree? Maybe, but a more troubling question for me is why nobody stopped this often incoherent, irrational young man on his long path to the rampage in Tucson. 

I don’t just mean the people who sold Loughner his Glock 19 semiautomatic pistol last November, or the people at a Wal-Mart who allegedly sold him ammunition a few hours before the assault. I mean the community in which he lived. This was a young man who showed signs of mental illness, yet in our culture people couldn’t or wouldn’t stop him ― even when they knew his behavior was bizarre.

We leave people alone in America, to a fault. We walk past rambling, dazed homeless people every day, if we live in big cities, avoiding their gaze rather than seeking to intervene. And even when we try to stop people whose behavior seems to pose a danger to themselves or others, it’s hard to do anything about it, as Loughner’s professors at Pima Community College discovered.

Look at the moon-faced grin of the alleged shooter as he appeared in court for arraignment Monday. It’s a haunting photo, not least because we have seen faces like that before ― people who are severely disturbed but on the streets in this era of “de-institutionalization.”

Here’s what I walked past this morning before sitting down to write this column. Outside my subway stop were two shopping carts, bearing what appeared to be the worldly possessions of two homeless people. They had fled on this bitterly cold day, but a distracted young woman was walking aimlessly nearby with a thin jacket, shivering. At a transit point, a ragged man with no teeth staggered out of the car. On another train, a woman talked incessantly to herself, as if in a trance. Nobody intervened. She didn’t look suspicious; just crazy.

If you’ve ever worked in a homeless shelter, you know that a substantial number of the residents have mental health issues, often combined with drug and alcohol problems. They might once have been in state asylums, out of sight and mostly out of mind. Those were nightmare places, and the de-institutionalization movement that put them on the streets was right, on balance. But the idea was to fund halfway houses and treatment, not just let people wander.

The Loughner case is telling in that it shows how hard it is to do something about an unstable person, even if you try. Pima Community College said that Loughner last year “had five contacts with PCC police for classroom and library disruptions.” Yet as Timothy Noah noted in Slate, he wasn’t suspended until Sept. 29, after he denounced the college as “a scam” and his teachers as “illiterate.”

Students and teachers knew that Loughner was potentially dangerous. His algebra teacher, Ben McGahee, told the Washington Post that when he complained about Loughner’s disruptions, “They just said, ‘Well, he hasn’t taken any action to hurt anyone. He hasn’t provoked anybody. He hasn’t brought any weapons to class.’” Loughner “scares me a bit,” wrote one of the students in an e-mail quoted by the Washintong Post. “Until he does something bad, you can’t do anything about him,” she wrote in a later message. “Needless to say, I sit by the door.”

“His thoughts were unrelated to anything in our world,” said his philosophy professor, Kent Slinker, in an interview with Slate’s Christopher Beam. He was, Slinker said, “someone whose brains were scrambled.” And yet he kept on rolling toward the Safeway parking lot, buying his gun, buying his ammo, leaving behind crazy YouTube videos that were advertisements of his madness.

The Tucson shootings have prompted a national debate about the decline of civility in America. That’s good, but we should expand the definition. A civil society isn’t just about less screaming on cable TV. It also has an ethic of community, so that people try, as best they can, to look out for one another.

There’s a coarsening, uncivil effect when we watch homeless people ranting and mumbling, freezing in the cold ― and cross the street, assuming that it’s somebody else’s business. It takes something out of us, individually and as a country.

Like most of the problems that matter, Jared Loughner didn’t sneak up on us and catch us by surprise. We saw him coming and didn’t do anything about it.

David Ignatius’ e-mail address is davidignatius@washpost.com ― Ed.

(Washington Post Writers Group)