The American anthrax probe was being stepped up after scientists discovered cases in New York and Florida were caused by the same strain of the deadly bacteria.
The discovery came as 31 workers at Congress were being treated for exposure to the bacteria and an investigation was under way into a find of the spores in the offices of New York governor George Pataki.
And it also came as fears of a co–ordinated bioterrorist attack on America mounted and suspicions increased that Osama bin Laden's al Qaida network was behind it.
The lower house of Congress has been suspended until Tuesday as police and public health officials launch an inch–by–inch examination of offices and mailrooms in a search for contamination after a letter sent to a senator was found to contain the deadly spores.
The first case of anthrax was revealed more than two weeks ago, at the offices of American Media Incorporated, a publisher of tabloid news magazines in Boca Raton, Florida.
In New York Erin O'Connor, 38, an assistant to NBC newsreader Tom Brokaw, was continuing to fight the disease, which she contracted from a letter sent to Mr Brokaw.
The discovery that the cases were caused by the same strain, by the American Centres for Disease Control and Prevention (CDC), is the first firm link between the two finds of the spores which cause the deadly disease.
The first outbreak was in Florida more than two weeks ago, where British–born journalist Bob Stevens, 63, died after inhaling the spores, and a mailroom worker at AMI, where the dead man worked, is being treated in hospital for the disease.
The discovery made after preliminary testing by the CDC is the first link between the two cases, which had until now been treated as separate but parallel investigations by the FBI.
Tests were also under way today on the anthrax discovered in Washington DC in a letter sent to Tom Daschle, leader of the majority Democrats in the Senate, which left 31 people exposed to the spores.
Preliminary tests determined that the anthrax was professionally made but was not weapons–grade bacteria and is responsive to antibiotics, a senior official said.
It has been linked to the NBC case because both came in letters franked in Trenton, New Jersey, and showed "similarities" in the handwriting, FBI investigators revealed.
Hundreds of staff at the Capitol building are being tested for anthrax, and many have been given the antibiotic Cipro.
But there was a split between the two houses of Congress as the Senate decided to keep sitting regardless of the scare and close offices only as long as was needed for them to be tested.
Mr Daschle said: "It is my strong determination that we will not let this stop the work of the Senate."
In New York, traces of anthrax were found in an office used by Governor George Pataki's squad of police who provide him with security.
Two floors of government offices were closed and up to 80 members of staff given antibiotics as a preventative move.
Mr Pataki said: "There has been no positive indication in any individual of anthrax. I am taking Cipro. The bottom line of this is that I feel great."
The outbreaks have sparked fear across America, with sales of Cipro, the antibiotic used to prevent the disease developing after exposure to the spores, rocketing and its producer trebling production to meet demand.
It has received the volume of orders it would receive in a month in just the past five days and is now going to produce 60 million tablets a month.
Suspicions that Osama bin Laden and al Qaida are behind the outbreak have increased, with a series of cabinet members saying they suspect the terror group is launching a biological attack.
But Mr Pataki said he did not want people to live in fear because of the anthrax outbreaks.
"It is obvious that we are engaged in a war – it is a war of nerves and terror," he said.
"These are evil people and we are going to drive these people back to the caves where we belong."