The September 11 terrorist attacks on New York and Washington have created a seemingly impossible challenge for the nation's architects: designing secure structures that still reflect a feeling of openess for the people who use them.
The clean up at the site in lower Manhattan where the World Trade Center once stood has been going on non-stop since the hijacked jetliners slammed into the landmark twin towers.
Authorities have not decided what if anything will be built in their place.
Three-hundred kilometers to the south, workers are filling in the gaping, blackened hole on one side of the Pentagon near Washington, DC.
The terrorist strikes on these U.S. symbols changed the routine in many cities. In New York, tourists now stand in silence at ground level, where the World Trade Towers were, instead of enjoying the view high atop the observation deck. In Washington tours of the Pentagon are suspended.
Visitors who used to soak in the history of the U.S. Capitol are now restricted from wandering the halls. Metal detectors, armed guards, and concrete barriers are now as common as gift shops.
Americans are demanding safer buildings and secure public spaces. The challenge for designers and engineers is building structures that do not look like armed fortresses.
Harold Adams is chief executive officer of RTKL, the architectural firm rebuilding the damaged Pentagon.
"Free and open access. Our government is based on it, our buildings and cities reflect it. Never have our leaders been behind castle walls," says Mr. Adams.
Many engineers say the technology exists to design buildings that can stand up to anything a terrorist can do, including bomb blasts and chemical attacks. They say the problem is that such buildings would not only be too expensive, but they would be too ugly or intimidating for anyone to want to live or work there.
Former New York Senator Daniel Patrick Moynihan was an advocate of many of the nation's most important urban building projects.
"We have to let the world know that we are not living in caves and have no intention to do so," the former Senator says.
At a Washington symposium on designing safer buildings, a panel of engineers and public safety experts talked about a number of ways to bolster security. They all agree that new structures can be graceful and elegant - open and free - and safe.
"Surely, ours must be openness and fearlessness in the face of those who hide in the darkness. A precaution, yes. Sequester, no," says Senator Moynihan.
Suggestions include rapid air extraction systems in case of a biological or chemical attack, special materials that do not react to chemicals, and metal detectors built directly into entrances and arches.
They also say local police and fire departments must be given plans to major buildings for search and rescue efforts.
Behavior expert Richard Farson believes builders should consider smaller facilities that do not need scary security measures, which he says make people suspicious of one another.
"Architecture is potentially more important in building a great society than education, or medicine, or any other profession," he says." And the reason for that is you design situations and situations are what determine behavior."
The panelists say terrorists can destroy buildings, but not democracy. And they say it is up to the nation's designers and engineers to help ensure this.