The government has not decided if it will try to locate tens of thousands of letters mailed around the country that were possibly tainted with anthrax in a New Jersey postal facility.

"With each passing day, the lack of further cases occurring is grounds to diminish the risk from any one of these letters," stressed Dr. Jeffrey Koplan, head of the Centers for Disease Control and Prevention in Atlanta.

Investigators already have tracked 300 letters that passed through the Trenton, N.J., facility within seconds of anthrax-laden attack letters mailed to Democratic Sens. Tom Daschle and Patrick Leahy. Health officials in every region that received the suspect letters are watching for anthrax symptoms, but so far no infections have turned up, Koplan said. 

But the way the mail was processed suggests far more than 300 letters could have picked up small amounts of anthrax in that post office, he said. 

"There seems to be the potential for not just hundreds and not just thousands but tens of thousands and maybe more letters to be potentially at risk for some level of cross-contamination," Koplan said. 

The CDC hasn't yet decided if this mail also needs special tracking and study. 

The count comes as investigators struggle to unravel the mystery of how two women — Ottilie Lundgren, 94, of Oxford, Conn., and Kathy Nguyen, 61, of New York — inhaled enough anthrax to kill them. Nguyen died in October; Lundgren, in November. 

No traces of the bacteria have been found in either woman's home, mail or usual locales. Yet suspicion that cross-contaminated mail might have killed Lundgren grew Monday because of circumstantial evidence: A trace of anthrax was found at a Connecticut postal center that processed mail for her hometown, as was a letter mailed to a nearby home that contained a single anthrax spore. 

Linking Nguyen to cross-contaminated mail is even harder. A letter addressed to a South Bronx business near her home went through the Trenton postal facility at about the same time as the letters to Daschle and Leahy, but that letter hasn't been found. 

The public shouldn't conclude that this was the way the women were infected, said Dr. D.A. Henderson, the government's top bioterrorism adviser. "We have difficulty in accepting that," because past studies have suggested trace amounts aren't enough to cause the deadly inhaled form of anthrax. 

Still, scientists do not know the minimum safe level of anthrax, and people with weak immune systems could be infected by far lower amounts than it would take to sicken healthier people. So Koplan said such at-risk people might consider having others open their mail, or hold it away from the face, as steps "prudent for their peace of mind." 

"People have a right to be uncomfortable about this. We don't have all the answers," he added. 

Meanwhile, investigators hope the Leahy letter will offer important clues to the source of the anthrax and the perpetrator, because it was discovered unopened and full of the bacteria. But they haven't yet determined how to open the letter safely. 

All the other known anthrax-attack letters were opened, letting the bacteria escape, before the investigation even began. In Washington, a Senate office building contaminated when the Daschle letter was opened Oct. 15 may not reopen until early next year, cleanup workers said Monday.

The Associated Press contributed to this report.