The US has entered an "unusual period in American history".
Postal workers up for anthrax in Washington
The White House has defended this week's national security alert by saying it is needed to protect areas and assets that might be vulnerable.
But in the face of threats that can turn a letter into a deadly weapon, just about anything is a vulnerable target these days. With even the House of Representatives recently forced to close for decontamination, the US capital is entering what the President this week referred to as an "unusual period in American history."
The US has fought many wars, but what Americans are now calling the "homeland" has not been attacked for nearly 200 years. In the wake of September 11, they are adapting to an unaccustomed sense of vulnerability.
Before the attacks, Washington - like other American cities - was riding on a wave of economic prosperity and urban safety. Now, the capital is facing a sharp drop in tourism, a jump in joblessness.and a bad case of the jitters.
This creates new challenges not only for American officials, but also for the foreign diplomats posted to a city under a new kind of siege.