The anthrax that closed down parts of the Capitol, and which almost certainly killed two postal employees, was a more pure and deadly version of the bacteria than that sent to media outlets in the US, authorities said.
The Homeland Security director, Tom Ridge, said the spores sent to the Senate Majority leader, Tom Daschle, were more dangerous, possibly because they were coated with an additive that helped to suspend them in the air, allowing them to be inhaled.
"It is highly concentrated, it is pure, and the spores are smaller. Therefore, they're more dangerous because they can be more easily absorbed in a person's respiratory system," said Mr Ridge.
The revelation altered a previous comment that the anthrax sent to Mr Daschle was "virtually indistinguishable" from that sent to media outlets in Florida and New York. Analysis of the letters that contained the anthrax has suggested a number of similarities.
The new information came as more locations in Washington were found to be infected with the bacterium, and as Mr Daschle announced that parts of the Senate building where it was discovered will have to be sealed indefinitely.
Mr Daschle said the entire south-east portion of the Hart building, opposite the Capitol, will have to be closed, shutting down the main offices of a dozen senators. He also revealed that a small number of spores had been found in a freight elevator area on the first floor of the building.
Mr Daschle said he did not know how the buildings at the centre of the anthrax infection would be sealed off. It will require special material because authorities have said the anthrax spores in the letter were barely more than a micron in size and could slip through most filters (there are 25,000 microns in an inch).
The ease with which the finely milled spores can travel was underlined by news that a female journalist who covers Congress had been taken to hospital with suspected anthrax. If confirmed, she would be the first person to have contracted the disease inside the Capitol complex since a letter sent to Mr Daschle and opened on 15 October exposed around 30 people to the bacterium.
The woman was outside Mr Daschle's office the day the letter was opened, and has complained of flu-like symptoms. She was treated at the same Maryland hospital where a number of employees from the Brentwood postal depot who have been infected – presumably from the now-notorious "Daschle letter" which passed through that distribution centre – are also being treated.
Dr John Eisold, the Capitol physician, said he had "good reason to believe that this person will turn out to be fine''.
Meanwhile, a second NBC employee in New York was diagnosed with cutaneous anthrax after handling the letter sent to the newsreader Tom Brokaw last month.
The Washington Post reported yesterday that experts had found the anthrax sent to Mr Daschle was coated with a chemical additive that helped the spores to remain suspended in the air. The report noted that only the US, Russia and Iraq had such technology.
Mr Ridge did not talk about the possible source of the anthrax, but his comments suggesting a more pure, "weaponised" form will add to fears of the involvement of an expert in the spate of mailings.
Despite the wave of anthrax infections, a poll has revealed that most people in the US still have faith in the safety of their mail. The ABC poll found that 90 per cent of people thought mail arriving at their home was safe, and while 75 per cent of people said they were generally concerned about the threat from the bacterium, only 10 per cent considered themselves scared.
* Security experts in Germany are investigating whether the hijack suspect Mohamed Atta carried anthrax spores allegedly obtained from Iraqi agents to the US.
The German tabloid Bild cited unnamed Israeli intelligence sources as saying that Atta, who is suspected of flying a plane which crashed into the World Trade Centre, received anthrax spores from Iraqi agents during two visits to the Czech Republic.Reuse content