The tainted mail was discovered as part of routine mail testing by the Fed in a mobile trailer stationed in a courtyard at the Federal Reserve's main buildings in downtown Washington. The test results were preliminary.
The mail did not contain powderlike substances or handwritten addresses. Such preliminary tests often are inaccurate and further tests are being conducted.
"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 said in a statement.
The mail that tested positive this week bore April and May postmarks and was addressed to a variety of Fed personnel, including some to Fed Chairman Alan Greenspan.
However, a U.S. official Thursday said there was a "very good likelihood" that the anthrax results could prove to be false positives or cross-contamination from earlier incidents.
"(There is) is a very good likelihood this is a false positive ... (or) cross-contamination," said the U.S. official, who spoke on condition that he not be identified.
A Fed spokesman said the spores were found in mail distributed from the Fed's external mail facility and added that later tests were negative for anthrax in the mailroom and mail distribution points within the Fed's three buildings.
While the Fed is still accepting mail, distribution was suspended on Wednesday.
Since October, the Fed has been testing its mail on a routine basis.
The Fed also had positive initial test readings for a batch of mail received in December. However extensive follow-up tests never found any new letters containing anthrax.