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But New Scientist can reveal that the bacteria used in the attacks is not a strain that Iraq or the former Soviet Union mass-produced for weapons. In fact, it is either the same strain the United States itself used to make anthrax weapons in the 1960s or close to it. Neither the strain nor the physical form in which it has been sent out is particularly sophisticated, say specialists.

What may matter more than the strain is how big a batch this anthrax came from. This could reveal not only how many more of these mailings we can expect but also whether the bacteria were brewed in small-scale, makeshift labs or bigger facilities. Work that could tell us is under way at a lab in the United States. Crucial geopolitical decisions could rest on what emerges from the electrophoresis gels and computer programs of the lab’s small band of geneticists. Last week, Tom Ridge, President Bush’s newly appointed Homeland Security adviser, stated that the anthrax sent to Florida, NBC, and Sen. Tom Daschle were all the same strain. An FBI spokesman in Florida confirmed the widespread reports that this was the Ames strain.

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But there has been confusion over what “Ames” means. The name was given to a strain isolated at the U.S. Department of Agriculture’s veterinary lab in Ames, Iowa, in the 1930s. This strain, which was later shared with microbiologists around the world, still strikes cattle in the western United States. Recent American military research publications also mention an Ames strain isolated from a cow in Iowa in 1980. However, the scientists analyzing the anthrax from the attacks are comparing its DNA with a library of strains collected from all over the world. And in this collection, what’s called Ames has more interesting origins. It emerged in the 1980s from a freezer for the Center for Applied Microbiology and Research, the British biodefense establishment at Porton Down, Wiltshire.

Porton Down acquired it from the U.S. Army Medical Research Institute of Infectious Diseases in Maryland. It is, say those who compiled the library, the strain the United States used when it produced anthrax weapons. That program ended in 1969, and the mass-produced anthrax was destroyed, although the United States and its allies kept samples. To be identified as “Ames” by these scientists, therefore, the anthrax used in the recent attacks must either be the American military strain or one that’s very similar.

So why choose this strain? “Ames is certainly a challenge to any vaccine,” says Martin Hugh-Jones of Louisiana State University at Baton Rouge. When lab animals immunized with the vaccine now being given to thousands of American troops are exposed to anthrax, many are still killed by the Ames strain.

Alternatively, the attackers may simply have wanted a strain of proven virulence that’s hard to trace, says Ken Alibek, former deputy head of the Soviet bioweapons program. “If I were a terrorist, I would certainly not use a strain known to be from my country,” he told New Scientist. The Soviets did not mass-produce Ames, says Alibek. Nor did the Iraqis. Like Britain in the 1940s, Iraq favored the Vollum strain, isolated in Oxford in 1930, which has been identified in samples from its Al Hakam plant. And the White House reiterated last week that all anthrax mass-produced in the United States was destroyed after 1969.

Despite this, Ames would not have been hard to find. Samples of the weapons strain were kept in the United States and elsewhere. Important clues also come from the size of the particles used in the attacks. According to reports last week, they had been milled down to a few micrometers, which is optimal for causing the inhalation form of the disease. “The terrorists at least had access to considerable know-how,” concludes Michael Powers of the Chemical and Biological Arms Control Institute in Washington, D.C. “This suggests some level of state involvement.”

But Alibek dismisses claims that milling the powder this fine is too hard for mere terrorists. His view is supported by a secret experiment last year called Project Bacchus, in which employees of the U.S. Department of Defense covertly produced a kilogram of bacteria similar to anthrax. It was milled to a few micrometers using machines available openly in the United States.

Nevertheless, the attacks have caused relatively few inhalation cases so far, which suggests that the spores were not blended with the anti-caking chemicals used to promote airborne spread. This is the secret of “weaponized” anthrax, says Alibek. He says sending anthrax in the mail is a “very primitive” way of distributing it and suspects the attackers don’t have much material.