Again and again, this country has asked itself: How can this happen?
We investigate. We ultimately get answers. And they almost always present themselves as a variation on a familiar recipe of poisons: mental illness that has gone untreated, a culture that has encouraged the basest forces of our natures, guns that have fallen too easily into the wrong hands, signals that were missed, laws and systems that have failed.
That a congresswoman could have been shot, and others murdered, simply because she was doing the work she had sworn to do has opened up a national debate over how we conduct our national debates.
Could decency have prevented this? We still don't know much about what allegedly drove Jared Loughner, 22, to fire a bullet into the head of Rep. Gabrielle Giffords, D-Ariz., and to shoot those around her, but it seems that his politics were not a big part of it.
Yet once the investigations are done and the grief and fear lose their edge, experience suggests that not all that much will really change. Learning an answer is not the same as learning a lesson.
While tragedy occasionally presents a political opportunity to one side or the other, that edge is rarely as sharp -- or as enduring -- as the deep philosophical and ideological divisions of a polarized nation.
In the wake of the shootings in Tucson, gun laws probably will remain pretty much as they are now. Mental health treatment will remain underfunded in cash-strapped states, the system for identifying potentially dangerous people will remain porous. If the vitriol does recede for a while, it will return.
And the next time, when something unthinkable happens, a shocked country will once again tell itself that everything has changed, until it turns out that it hasn't.
For a while in the late 1990s, it seemed that every few months brought news of another horrific shooting in a school. They were all the more shocking because they happened in so many I-never-thought-it-could-happen-here places -- West Paducah, Ky.; Jonesboro, Ark.; Littleton, Colo.
At one point then, as President Bill Clinton was preparing to meet with yet another group of grieving loved ones, he turned to Bruce Reed, his top domestic policy adviser.
"I can't go in there and tell them it's going to be different," Reed recalls his boss saying that day, "because I've been through this too many times."
There have been moments, of course, when tragedy has rewritten history's story line.
The justness of the civil rights movement was all the more undeniable for the fact that it had cost some advocates their lives in the 1950s and 1960s. The assassinations of Martin Luther King Jr. and Robert F. Kennedy in 1968 brought about the first major revisions in the nation's gun laws in 30 years.
But more recently, the aftereffects and opportunities of national horror have been shallow and brief.
In April 1995 -- at a political moment not unlike this one -- anti-government fury took on a new face when Timothy McVeigh blew up the Alfred P. Murrah Federal Building in Oklahoma City. He killed 168 people, the majority of them government workers, and also 19 children.
Clinton -- who only a day before the bombing had been reduced to pleading that "the president is still relevant here" against the new Republican majority in Congress -- reclaimed his own stature and that of his office with his handling of the moment.
"We hear so many loud and angry voices in America today whose sole goal seems to be to try to keep some people as paranoid as possible and the rest of us all torn up and upset with each other," Clinton said. "They spread hate. They leave the impression that, by their very words, that violence is acceptable."
Those words still rankle conservatives, who saw them as the exploitation of a tragedy to discredit his Republican opponents. And though Clinton's own standing rose again, his package of anti-terrorism legislation, introduced before the bombing, was nonetheless watered down by Congress. And only seven months later, the two parties were at odds in a government shutdown.
A month after the 1999 tragedy at Columbine High School, Vice President Al Gore strode into the Senate chamber to break a tie on a legislation to limit access to guns and put trigger locks on them.
His position ultimately cost him votes in crucial swing states. These days, Democratic campaign ads are more likely to feature one of their candidates holding a gun than advocating restrictions on them.
Nor is it clear that either side in this deeply divided political culture will benefit in the long run from the tragedy in Tucson.
"Anybody who looks for political advantage in this is missing the whole point," said Reed. "This is a blow against democracy. It may change the tenor of the political debate in a way we can't see, but it's crazy to point fingers. It ought to make everybody look deep inside themselves."