Traces of anthrax have been found again on a mail sorting machine that tested positive in October, the US Postal Service has confirmed.
The machine, at the Morgan Processing and Distribution Centre, had been testing negative since October, but a new round of tests conducted on December 23 came back positive on one machine Friday night.
Although the city's health commissioner said there was no health threat, the president of the New York Metro Area Postal Union said he would tell employees not to return to work until the extent of the contamination was clear.
It was not immediately clear whether any of the more than five and a half thousand workers had refused to return to the building.
Postal officials said that despite widespread testing in the facility - which handles 12 and a half (M) million pieces of mail each day - the third-floor machine was the only one to test positive.
The machine will be cleaned and retested, but the facility will not be closed.
After the first contamination in October, postal officials sealed off isolated areas in the facility for decontamination
Shortly afterward, union officials filed a lawsuit seeking to shut the entire facility until the anthrax had been removed, but a judge denied their request.
No case of anthrax has been confirmed among New York's postal workers, but mail that likely went through Morgan is blamed for four cases of skin anthrax in the city, all of them non-lethal.
A New York hospital worker died on October 31 of inhalation anthrax, but officials haven't determined the source.
Meanwhile, workers pumping poison gas into a Capitol Hill office building hope to finish the job by mid-morning.
Saturday was the third day in the latest attempt to kill anthrax spores that remain in the Hart Senate Office Building.
The building has been closed since mid-October, when a tainted letter addressed to Senate Majority Leader Tom Daschle was opened.
Environmental experts hope this will be the final fumigation using toxic gas.
Once technicians are finished, they say another three days of testing will be needed before the building can be declared safe.