Anthrax sent to the US Senate was treated with a chemical only three nations, including Iraq, are thought to be capable of making, it was revealed today.
The spores in a letter sent to US Senator Mr Tom Daschle, leader of the majority Democrats in the Senate, had been treated with the chemical to enable them to stay in the air, making them easier to inhale and therefore deadlier, the Washington Postreported.
An official close to the investigation told the newspaper only the US, the Soviet Union and Iraq had been known to have developed the additives.
And there is hope that because each of the three countries used slightly different techniques, microscopic and chemical analysis might show up where the chemical was from.
But experts have warned that it is unclear how secure the former Soviet Union's or Iraq's supplies are, and it is also possible their scientists could have defected elsewhere.
The revelation came as the New York Postreported that the FBI may be concentrating on homegrown terrorists, including right-wing extremists.
Investigators are looking at at least 12 other letters sent to media outlets, which did not contain anthrax, but had similar messages and hand-writing as the letters laced with the deadly spores.
Hoax letters are also being investigated to see if some are from the same person or group.
A source close to the investigation said: "Our feeling is the anthrax does not point to an international group.
"The only way it could be is if they are purposely writing letters that point away from them as a ruse and using anthrax that we believe was manufactured here."
PA