The US Federal Reserve says about 20 pieces of mail have tested positive for traces of anthrax in an initial screening.
The tainted mail was discovered as part of routine mail testing by the Federal Reserve.
It was in a mobile trailer stationed in a courtyard at the Federal Reserve's main buildings in Washington. The test results are preliminary.
The mail did not contain powder-like substances or handwritten addresses. Such preliminary tests often are inaccurate and further tests are being conducted.
In a statement, the Federal Reserve said: "The affected mail was routine commercial and business mail and did not have any of the characteristics identified by the FBI as suspicious."
The Fed did not have information on where the mail originated or where it had been processed.
All mail addressed to the federal government is irradiated to kill any possible anthrax but even the presence of dead spores could produce an initial positive reading.
Some of the mail was addressed to Federal Reserve Chairman Mr Alan Greenspan as well as other officials, Federal Reserve spokesman Mr David Skidmore said.
Mr Skidmore said the 20 pieces of mail were "fairly recent" with postmarks of April or May. Swabs that produced the positive readings will be sent to a laboratory for additional testing.