Government health officials investigating the anthrax attacks are facing growing criticism for making safety assurances that later were recanted and have ended up eroding the public trust.

The controversy came to a head this week when officials from the Centers for Disease Control and Prevention warned that tens of thousands of pieces of mail might have been cross-contaminated at a New Jersey postal facility that had processed the anthrax-laden letters to Democratic Sens. Patrick J. Leahy of Vermont and Tom Daschle of South Dakota.

That conclusion stemmed from the discovery last week of a letter in Seymour, Conn., that appears to have been contaminated while being processed shortly after the Leahy letter.

After assuring Americans for weeks that their mail was safe, postal and public health officials are exploring whether similar cross-contamination was a factor in the deaths of Ottilie W. Lundgren, 94, of Oxford, Conn., and Kathy T. Nguyen, 61, of New York. Cross-contamination is also believed to have caused a less serious case of skin anthrax in a New Jersey accountant.

"There are rules in public health about dealing with these kinds of things, and they seem to have broken every one of them," said Helen Schauffler, director of the Center of Health and Public Policy Studies at UC Berkeley. "You don't speculate about what might be happening. You don't falsely reassure. It only undermines people's confidence."

Meanwhile, Rep. Christopher H. Smith (R-N.J.) called upon government officials to release a list of ZIP codes that received mail processed at the Hamilton, N.J., site at the same time as the contaminated letters.

"It is possible that other letters could have been cross-contaminated and that other postal customers may be exposed to the bacteria," Smith wrote in a letter Tuesday to FBI Director Robert S. Mueller. "It may be in everyone's best interest that [ZIP codes be disclosed] so people are cautioned of this possible danger, however small the chances may be."

CDC officials said they are discussing how or whether they will notify Americans who might have received cross-contaminated mail but have made no final decision.

They stressed they still believe that the risk of infection caused by cross-contamination remains low.

But the agency is sufficiently concerned about the possibility that it plans to release an advisory to public health professionals nationwide, alerting them to the risk of contracting inhalation anthrax through cross-contaminated mail, said CDC spokesman Tom Skinner. Among other recommendations, the CDC suggests that Americans not hold mail near their faces and that those with health problems such as emphysema take extra precautions when opening mail.

The advisory is the latest in the CDC's evolving guidelines for coping with the anthrax attacks.

Schauffler credits the CDC for its rapid response to the anthrax crisis and aggressive recommendations on prescribing antibiotics, which she said probably saved lives.

But she worries that the repeatedly false assurances may be creating a credibility gap with an anxious public.

"It undermines the public's confidence," Schauffler said. "Public health can't be effective without the public trust. If we have another major crisis, the public may not believe what they are hearing or may not respond appropriately."

Jeffrey Koplan, the CDC's director, stresses that health officials are doing the best they can to cope with an unprecedented bioterrorist attack. He notes that the anthrax mailings mark the first time U.S. health officials have had to cope with the deliberate release of the deadly bacteria, which in these cases had been treated to make spores easier to spread.

"During the course of this investigation, we're learning things on an ongoing basis," Koplan said.

Government health officials said they do not think their efforts to calm and inform Americans have undermined public confidence or caused long-term damage.

"The public has a firm understanding that this is an entirely new ballgame," said Tony Jewell, a spokesman for the U.S. Department of Health and Human Services.

But public confusion over the government's response is understandable, experts say.