NEW YORK (CNN/Money) - Traces of anthrax have been found at an off-site Federal Reserve mail facility, the central bank said Thursday.

Routine, preliminary tests Tuesday and Wednesday found anthrax spores from an unknown source in about 20 pieces of commercial and business mail, the central bank said.

"The affected mail ... did not have any of the characteristics identified by the FBI as suspicious," the Fed said in a statement.

Officials said the preliminary positive could turn out to be incorrect, since preliminary tests are sometimes proven to be false, or could be due to "cross-contamination" from other sources.

According to the Associated Press, some pieces of mail were addressed to Fed Chairman Alan Greenspan, and all had postmarks from April or May.

The Dow Jones industrial average dropped about 70 points after the Fed released its statement, but it has since recovered.

Several anthrax-tainted letters were sent in the U.S. mail to government and media officials last fall, resulting in five deaths. A trace of anthrax was found at the Fed last December, but that was believed to have been the result of cross-contamination, and extensive follow-up testing yielded no further positive results.

The swabs with the positive readings will go to a laboratory for additional testing, and further tests of mailroom surfaces and mail distribution points were negative, the Fed said.

Fed officials are working with the FBI and U.S. Postal Service inspectors in the case.

The AP said distribution from the Fed's mail facility was suspended Wednesday, though the facility, located in a mobile trailer in a courtyard of the Fed's headquarters in Washington, D.C., is still receiving mail.