The death yesterday of a New York hospital worker from inhaling anthrax has sent a new shock wave through this country, as the 61-year-old woman's infection has mystified investigators. The White House expressed concern at the development.
Ms Kathy Nguyen, the country's fourth anthrax fatality and New York's first since September 11th, is not known to have visited a postal centre, or to have had any connections to media organisations or politicians who until now have been the targets of attacks. Her workplace has not tested positive for anthrax so far.
That reality is forcing the authorities once again to radically re-examine assumptions about the infectivity of anthrax, particularly of a highly volatile batch sent from New Jersey to the Senate majority leader, Mr Tom Daschle. Is it possible, they are asking, for the contamination to spread from envelope to envelope as they pass through the postal system, potentially exposing anyone who receives a letter? Were there other contaminated envelopes?
When the Daschle letter was received it was assumed that postal workers were not threatened by it. Two of them have since died and many have been contaminated, leading to strong criticism of the authorities' advice.
And they are having to consider the possibility that we are about to see new targets or the use of a new delivery system, although the assumption is still that the latest case is mail-related in some way.
"It is unclear whether this particular instance is part of a pattern of other cases or whether it represents something different," Dr Jeffrey Koplan, the head of the Centres for Disease Control, has confessed.
Investigators are trying to retrace the steps of Ms Nguyen, a single Vietnamese immigrant who lived alone and worked in the basement of a small ear, nose and throat Manhattan hospital, who went to hospital a week ago and was too ill to be questioned.
Up to 2,000 hospital workers, patients and visitors who have been to the hospital since October 11th are being offered antibiotics. The hospital has been closed and other hospitals in the city alerted to take precautions.
The White House spokesman, Mr Ari Fleischer, said preliminary tests on Ms Nguyen's home were also negative.
Ms Nguyen left Vietnam 24 years ago penniless. She came to the US with the aid of a soldier who was a police officer. Neighbours and co-workers said she loved to cook and shop. She was divorced and regarded her neighbours as her family since her only son died in a car crash years ago.
The postal service yesterday said an employee at a second regional mail facility in New Jersey was suspected of having the less threatening cutaneous (skin) anthrax.
The latest victims raised the number of confirmed anthrax cases to 17 nationwide. Ten have the inhaled form, including the four who died. The others have cutaneous infections and four of these - and two more suspected cases - are linked to city media outlets.
In Washington a congressman, Mr Mike Pence, told AP he had been told by investigators that the letter sent to Senator Daschle's office carried about two grams of anthrax. If the two grams were pure anthrax, it would contain enough spores to sicken about two million people, said Dr David Sullivan, an anthrax expert at John Hopkins University.