There have now been four confirmed cases of anthrax in the United States, one of which has proved fatal.
All four appear to have been caused by contaminated letters sent through the US postal system.
On Monday, the Washington office of leading US Senator Tom Daschle received a letter which has tested positive for anthrax.
Officials have said that the anthrax found in the letter appeared to be one of its most "potent" forms and most likely the work of experts.
Anthrax cases so far
American government sources said that preliminary testing had indicated the contaminated powder to be refined enough to disperse easily through the air, increasing its potential as a deadly weapon.
Investigators say there are hints but no hard evidence as to where the anthrax could have come from.
The FBI has said that the handwriting was the same on the letter sent to Mr Daschle and another sent to NBC news anchor Tom Brokaw.
They both contained the messages "Death to America" and "God is great".
Both letters were sent from Trenton, New Jersey.
Investigators are watching CCTV videotapes and interviewing postal workers to try and establish where exactly the letters were posted.
But with 46 different post offices routing mail through the city's main sorting centre, officials say they may never know precisely where the letter came from.
Postal workers have been tested for anthrax, but no cases have yet been found.
In Washington, about 50 members of Mr Daschle's staff and security personnel are being treated for anthrax, as a precaution.
No hard evidence
President George W Bush has pointed the finger at Osama Bin Laden, whose al-Qaeda network has already been blamed for the 11 September terror attacks.
"There may be some possible link. We have no hard data yet but it's clear that Mr Bin Laden is an evil man," said Mr Bush.
But he was unable to offer any specific link between the anthrax letters and Bin Laden.
"I wouldn't put it past him, but we don't have hard evidence yet," the president said.
The two letters posted in Trenton were postmarked 18 September, and it is not clear why they took so long to reach their destination.
The US Postal Service has set up a mail security task force, and issued guidelines to the public to be suspicious of:
Scares spread
In the wake of the Daschle letter, Capitol Police at the US Congress reportedly received at least 12 alarms on Monday from offices with suspect letters.
They all proved false.
In New York City, police were called to inspect over 100 suspicious packages in a single day.
On Monday evening, a Continental Airlines plane arriving from Las Vegas was quarantined at Cleveland airport with all passengers and crew on board, until a white powdery substance was cleared as non-toxic.
In addition to the four confirmed cases of disease, another 11 people have tested positive for exposure to anthrax, but have yet to develop symptoms.
The only fatality so far - a picture editor at the Boca Raton Sun - died of respiratory anthrax, the most serious form in which the spores are inhaled and lodge in the lung.
Anthrax spores small enough to be inhaled and to cause respiratory anthrax are the most difficult to manufacture in a laboratory.
A second employee at the same office in Boca Raton has also been diagnosed with respiratory anthrax.
An employee at NBC television in New York City contracted the less serious cutaneous anthrax, in which the bacteria enter via a cut in the skin, from a package addressed to newsreader Tom Brokaw.
The baby son of a producer at ABC News in New York City also tested positive for cutaneous anthrax, after visiting the building on 28 September.
However, it is not certain that the child contracted the illness there.
Broader investigation
Only one anthrax letter has come from outside the United States.
It was sent from Malaysia to a Microsoft office in Reno, Nevada.
The envelope, apparently returned to Microsoft from a retailer, contained pornographic pictures - one of which tested positive for anthrax.
Malaysian Foreign Minister Syed Hamid Albar said that the envelope might have originated in the United States, and simply have been posted in Malaysia.
"Malaysia looks at it very seriously and we are not a country that produces anthrax," he said.
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