Anthrax has turned up on mail destined for the U.S. Federal Reserve Board in Washington, D.C.

The Fed announced Thursday that mail processed in a secure facility outside the Federal Reserve Board buildings has tested positive for traces of anthrax DNA.

“Routine preliminary tests, administered late Tuesday and Wednesday by Board and contract employees wearing protective environmental suits, detected anthrax spores in small batches of mail totaling about twenty pieces. The affected mail was routine commercial and business mail and did not have any of the characteristics identified by the FBI as suspicious.”

The source of the contamination is not currently known.

An anthrax trace found at the Board last December is believed to have been the result of cross-contamination. Extensive follow-up testing after that incident yielded no further positive results.

The Fed says that in light of the new discovery, mailroom surfaces and mail-distribution points within the Board’s buildings have been tested, and all the results have been negative. Board officials and are working with the FBI and with U.S. Postal Service inspectors.