But the anthrax-tainted letters, while terrifying, have not been much of a test of the country's hospital network.

By comparison, the anthrax-tainted letters that killed 5 people and put 30,000 Americans on preventive antibiotics in 2001 each contained about a gram of dried anthrax spores.

The center, which handles mail for 46 area post offices, has been closed since October, after handling anthrax-tainted letters.

The anthrax-tainted letters, which killed 5 people, infected 18 and put 30,000 Americans on antibiotics, were the first significant biological attack in the United States.

Only three anthrax-tainted letters have been found; officials do not know if there are others.

A detective and two lab technicians who investigated an anthrax-tainted letter at NBC this month tested positive for exposure.

He was serving as mayor during the 2001 anthrax attacks when the regional postal facility located in the community received and distributed anthrax-tainted letters.

The additional precautions were instituted after anthrax-tainted letters began circulating in the mail, killing five people.

If no one at the mail stations opened any anthrax-tainted letters, how did the infections occur?

The discovery of a fourth anthrax-tainted letter was not a surprise to government epidemiologists, who have suspected that there must be other contaminated mail.