The images of Sept. 11, 2001 still shock, still sadden, still sear our souls: The awesome horror of a jetliner knifing into a skyscraper. The cruel beauty of black-red fireballs and fluttering debris against the blue late-summer sky. Ash-covered firefighters and police officers charging selflessly into the inferno. Helpless jumpers plunging silently into eternity. A tower dissolving story by story into itself, leaving part of its faÁade crumpled into a cockeyed cathedral.

Today, as we mark the second anniversary of that tragic day, it is right that we recall those images and mourn anew the 2,792 who died when two planes struck the twin towers of New York's World Trade Center. The 184 who perished at the Pentagon. The 40 passengers who crashed into a field in Pennsylvania, foiling terrorists from reaching their target - the U.S. Capitol or perhaps the White House.

And it is right that we celebrate the heroism of Sept. 11, how the very worst of days brought out the very best in Americans.

But two years have passed, and we have gone about our lives. On this Sept. 11, 2003, we risk losing sight of the tragedy's meaning, message - and imperatives.

We cannot forget. We must not forget.

To do so would dim the meaning and message of those deaths. It would put our way of life in greater peril.

Conventional wisdom tells us that Sept. 11 changed our nation - indeed, the world - forever. That is true. Yet at the same time, in ominous ways, it is not true.

The terrorist attack brought Americans closer together, revived our sense of national identity and purpose as other crises - Pearl Harbor, the JFK assassination - have done. Flags, banners, signs and stickers blossomed in a national show of renewed patriotism, which has since faded but not disappeared.

Americans rediscovered the backbone and determination that have blessed this nation - fortunately so, because the attack deepened an economic downturn and made recovery more difficult.

Perhaps most important, it changed American foreign policy. George W. Bush quickly declared a global war on terrorism as the focus of his presidency and the key conflict of the new century. This led us into Afghanistan and, for better or worse, into Iraq.

At home, it brought a system of alerts, heightened scrutiny, and tougher security at airports and public buildings. It rooted out cells of terrorist sympathizers. It ushered in a new phrase - homeland security - and a Cabinet-level department. It started civic dialogues over the meaning of Islam, religious and cultural tolerance, immigration policy, civil liberties and due process of law.

But much has not changed.

America has lapsed into the old, familiar patterns of a complacent, celebrity-driven consumer society. While focusing on passenger jets, we have all but ignored other potential terrorist vehicles, from cargo planes to trains to ships. Even at airports, we have delayed installing screening equipment. We have delayed enhanced passport security.

We tolerate security gaps, gaffes and breaches. We shrug at accounts of easily obtained fake IDs, and consider issuing genuine ones to illegal aliens. And partisan squabbling in Washington makes slow work of any change.

As many observers warn, a major terrorist attack could still happen here.

And yet it hasn't. That says something. America did not react the way terrorists perhaps expected, withdrawing into itself. It went on the offensive. It deposed the Taliban in Afghanistan. It disrupted and scattered the al-Qaida terrorist network. It toppled a dictator in Iraq, putting in motion a chain of events that could change the entire region. As supporters of the Bush policy say, we are fighting the terrorists over there so we won't be fighting them here.

Still, we remain vulnerable, as an open society inevitably is. A new video purportedly of Osama bin Laden was released Wednesday, timed for the anniversary. On an accompanying audiotape, the speaker has a chilling message for America: "The true epic has not begun. Brace yourselves for the punishment for your crimes."

The message has a subtext. As previous generations of Americans have learned, safety, stability and comfort can be elusive. Complacency is deadly. Freedom is not free. It is a dangerous world, and we are targets.

Even as the scars heal, even as lower Manhattan bustles with a new $10 billion project to replace the World Trade Center, we remain at risk. We face the task of ensuring that the ideals that define us - notions of liberty, equality, free expression, and individual dignity - prevail.

The images of Sept. 11, 2001 may fade, and its story may recede into the cool continuum of history. But the challenges with which it confronted America remain.

We cannot forget. We must not forget.