The anthrax used in the attacks on US media and political figures almost certainly originated in stocks developed by the US army, scientists here increasingly believe.
Genetic testing of the anthrax sent through the mail has been found to be genetically identical to that developed by the army at its Fort Dettrick, Maryland, US Army Medical Research Institute of Infectious Diseases (USAMRID) from the relatively commonly available "Ames" strain, the Washington Post claimed yesterday.
The findings should contribute a significant boost to a huge and, to date, largely fruitless investigation which has been increasingly coming around to the idea of a solo American opportunist with a grudge.
So far, only five laboratories have been identified as using the Fort Dettrick strain, the paper says.
They include the British research facility at Porton Down and Dugway Proving Ground, a military test facility in Utah.
The Dugway lab is the only one, the paper says, which is specifically known in recent years to have produced the highly volatile, milled form of anthrax which proved such an effective contaminant in the postal attacks.
The attack strain has been linked for some time to the Ames strain that was first isolated from an Iowa cow in the 1950s and has been widely distributed to research facilities in the US and abroad.
But it comes in various subtypes whose precise identification is only possible through genetic analysis.
That comparative work has been done by a group at the Northern Arizona University. "It's good detective work," Dr Jennie Hunter-Cevera, a microbiologist at the University of Maryland, told the Post.
"This will narrow the search for the people who had access to the strain." The paper also reports that investigators are looking at a secret research programme involving anthrax run by the CIA. A spokesman for the CIA has insisted that the agency did not develop the Ames strain or produce refined versions of bacteria and could not have been a source for the letters.
Meanwhile up to 3,000 people who have been exposed to the bacteria, including several hundred Washington postal workers and Capitol employees, may be asked to take a vaccine this week because of fears that they may retain anthrax in their bodies despite treatment with antibiotics.
The proposal, backed by local health officials, is likely to prove controversial as the vaccine has not yet been approved by the Food and Drug Administration and its potential use is be described as "experimental".
Some 75 Capitol workers, exposed when a letter to the Senate Majority leader, Mr Tom Daschle, was opened and contaminated several rooms, are reported to have had particularly heavy concentrations of spores in their bodies.
They will probably also be told by doctors to continue their antibiotic treatment beyond the 60 day course which had been traditionally thought of as sufficient.
The problem is that, with only 18 cases of inhalation anthrax in the US this century, experience of treatment is very limited.
Studies with animals show that in some cases, while antibiotics hold the disease at bay, one percent of spores can survive more than 75 days and become lethally active once the course is complete.
A combination of antibiotics and vaccination is said to be effective, but the vaccines have not been licensed yet for humans for post-infection use.
Health officials are also concerned that some of the postal workers have given up on the powerful antibiotics, cipro and doxicyclene, ahead of the 60 days, complaining of their side effects.
To date five people have died of inhalation anthrax in the current attacks.
There have been no new cases for several weeks.
