Federal health officials said yesterday that they are investigating the possibility of the anthrax threat spreading to homes, because of disturbing questions raised by infections in a New York hospital worker and a New Jersey accountant.

The new illnesses are challenging officials' theories about who might be vulnerable to bioterrorism, because the two women did not work for the U.S. Postal Service, government or news media, as have others sickened by the bacteria.

Investigators had believed that the outbreak of the rare disease in humans - 16 confirmed cases so far - could be explained by terrorists sending letters filled with anthrax spores to Congress and journalists. Postal workers may have become ill handling the letters, the theory went.

But Dr. Anthony S. Fauci, a top official at the National Institutes of Health, said yesterday that health officials are re-examining their assumption that the public stands little chance of contracting anthrax through mail delivered to their homes.

"Until yesterday, there was no evidence that there could be or is an individual in which there might be the reasonable question: 'Did they get infected from a piece of mail that went to their home?'" Fauci said at a news briefing. "That is being intensively investigated right now."

Tom Ridge, the U.S. director of homeland security, said yesterday that he believes that home delivery of mail poses little risk for most Americans.

This is especially true, Ridge said, if the public follows the Postal Service's advice to put aside any suspicious-looking mail and wash one's hands.

"We still think ... you ought to open your mail and you ought to use the postal system," Ridge said.

Twenty-five days after a photo editor at a Florida tabloid died as the first of three fatalities caused by the anthrax attacks, federal officials still do not know who spread the lethal bacteria or how it has turned up in so many government buildings.

In the District of Columbia, authorities said yesterday that investigators have found traces of anthrax in another post office - adding to a growing list of contaminated buildings.

The Friendship Post Office, at 4005 Wisconsin Ave. NW, received mail from the Brentwood mail-sorting center where investigators found anthrax this month. Four postal workers at the Brentwood facility were sickened with inhaled anthrax, two fatally. The center handled an anthrax-tainted letter sent to Senate Majority Leader Tom Daschle of South Dakota.

Along with the U.S. Supreme Court, the Hart Senate Office Building and other government facilities, the Friendship Post Office will be closed until health workers decontaminate it, officials said.

"It is exceedingly clear that the target of the terrorists in this situation was the three branches of government," said Dr. Patrick Meehan of the federal Centers for Disease Control and Prevention in Atlanta.

Traces of anthrax have also been found at a retail facility near Dulles International Airport in Virginia and in the mailroom of the U.S. Department of Agriculture Economic Research Service in Washington, officials said.

A puzzling case in N.Y.

In New York, investigators are trying to trace the footsteps of the most recent anthrax victim, a 61-year-old stockroom worker at Manhattan Eye, Ear and Throat Hospital.

The woman, whose name was not released, felt chills and muscle aches Thursday. She went to Lenox Hill Hospital on Sunday, complaining of severe breathing problems, according to the New York City Department of Health.

"The woman is critically ill," City Health Commissioner Neal Cohen said at a news conference yesterday. "There is evidence that the inhalational anthrax has released a lot of toxins and done a lot of damage systemically, and at this point she is struggling for survival."

Her case is puzzling because investigators have not found an anthrax-laced letter that she might have opened, and she only occasionally handled mail as part of her job, health authorities said.