Traces of anthrax have been found on a package sent from the NBC TV network to the New York mayor's office.
Spores have also turned up at a Veterans Affairs' hospital in Washington.
The anthrax reported in New York was "cross-contamination", meaning the spores originated from another tainted letter - in this case mail sent to NBC last month, officials said.
The Washington hospital could also have received spores indirectly, from mail processed at a postal centre that handled an anthrax letter sent to a top US senator.
Since last month, 10 Americans have developed inhaled anthrax, the most serious form of the disease. Four people have died. Those infected include postal and office workers believed to have come in contact with contaminated letters.
PA