US Health Chief Thompson said on Friday he did not expect analysis to confirm the presence of anthrax at the US navy mail handling centre, which has been shut down following the scare.
"Our preliminary analysis is underwhelmed," Thompson said.
The US Postal Service has shut down 11 facilities in the Washington area after preliminary tests indicated possible anthrax at a US navy mail handling centre.
The navy closed the automated mail handling operation at its naval air station in Washington on Thursday to run additional tests after sensors detected traces of a substance that could be anthrax, a navy spokeswoman said.
But Thompson struck a cautious note in giving the clean chit, insisting he was only giving his personal opinion.
"Our preliminary analysis is underwhelmed"
US health chief
"They have closed down postal facilities for protection. But we have had false positives before and this one could be or could not be, we wont know for another 24 hours," he said.
The suspicious substance was identified late on Wednesday by equipment that routinely samples the air in the facility and preliminary tests indicated that it could possibly be anthrax, a deadly bacterial disease.
As a precaution, the US Postal Service closed 11 facilities that were serviced by the same contractor that transported mail to the navy centre, a spokesman said.
"Out of an abundance of caution we want to make sure we've covered everything," spokesman Mark Saunders said. "We learned late Thursday the navy test was positive and that more samples were being taken at the facility."
Saunders said that the Postal Service would also test its
closed facilities. The shutdown affects about 1500 postal
In 2001 a series of anthrax attacks, which spanned the East Coast from Florida to Connecticut, killed at least five people, including two Washington postal workers.
Five fatalities recorded in 2001 -
including two postal workers
Letters containing anthrax spores were mailed to news media offices in New York and Florida and anthrax-laced letters were sent to the Washington offices of two senators.
The FBI was accused of a cover-up in not investigating a big lead - a scientist working for a firm contracted to produce anthrax for the CIA - for fear it would expose the US' secret biological weapons' programme.
Since the anthrax attacks, the Postal Service has spent
millions of dollars to test and install equipment to sterilise
Anthrax is a bacteria that causes disease in livestock and
in humans who have contact with animals. It is considered the No 1 threat for a biological weapon because it is deadly and
the bacteria form spores that are hard to kill.