A West Trenton, N.J., mail carrier whose case of skin anthrax led the FBI to theorize that she had picked up terrorist letters along her route may have gotten the disease instead by delivering mail that had been cross-contaminated elsewhere in the postal system, investigators said yesterday.

Using processing codes imprinted on some envelopes and packages, postal inspectors have traced the paths of the three letters known to have contained anthrax spores. They have been able to pinpoint how the letters' passage through the system contaminated numerous machines and have provided new evidence that other mail probably picked up spores from tainted equipment.

Within three hours of each other on Sept. 18, two of the terrorist letters were postmarked by the same machine at a postal center in Hamilton Township, N.J., and then passed through two sorting machines, outbound to the editor of the New York Post and to NBC News anchor Tom Brokaw in New York. Those sorting machines eventually tested positive for anthrax spores, the officials said, and they now believe the sorters might have transmitted spores to inbound mail as it was segregated at Hamilton for distribution in the Trenton area.

When mail carrier Teresa Heller was diagnosed with cutaneous anthrax several weeks ago, investigators thought she might have handled one or more terrorist letters before they reached Hamilton, and they began an extensive search for possible suspects along her route.

None were found, and postal officials now think Heller might have become infected by mail that had picked up spores at Hamilton before being sent to the West Trenton Post Office, where she gets letters and packages for her route.

If Heller did become ill that way, it is possible she delivered contaminated mail, although no illnesses have been reported along her route.

In addition to helping with the Heller case, the letter codes, which are sprayed on the back of some letters during processing, have enabled postal investigators to get a more precise idea of which letters caused each of nine postal workers to contract either cutaneous or inhalation anthrax. Two postal workers died.

"We learned a lot," John Nolan, deputy postmaster general, said at yesterday's briefing at the U.S. Postal Service's L'Enfant Plaza headquarters.

Together, the Post and Brokaw letters might have been responsible for the illnesses of Heller and a Hamilton worker who does routine maintenance on postmarking machines. Two of those machines tested positive for anthrax spores, and the maintenance worker was on duty the evening the Post and Brokaw letters were postmarked. He also had a cut, providing an entry point for bacteria.

Nolan said investigators learned even more from the third letter, which passed through the Hamilton facility on Oct. 9 on its way to the Hart Senate Office Building and the office of Senate Majority Leader Thomas A. Daschle (D-S.D.). That letter might be responsible for seven illnesses: three people at Hamilton who work in the sorting area, two of whom contracted inhalation anthrax and the other, cutaneous; and four workers at Washington's Brentwood processing facility, all of whom contracted inhalation anthrax.

Investigators know now that the Daschle letter went through Sorter No. 17 at Brentwood. At the time, such machines were routinely cleaned by using compressed air, and that cleaning created air currents within the cavernous Brentwood facility. Tests found anthrax spores at numerous sorters "downwind" of No. 17.

Three of the stricken Brentwood men worked in the sorting area, including Joseph Curseen, one of the two men who died. The fourth, Thomas Morris, who also died, worked in a section of the facility that handles mail destined for federal offices. Officials said it is possible Morris became infected when he rifled trays of government mail, a routine procedure to ensure that letters have been placed in the right tray.

Although the postal service's effort found possible explanations for the illnesses of its workers, it did not clear up how three other individuals might have become sick. One is a Hamilton Township resident who developed cutaneous anthrax but has no connection to the postal service, media outlets or government offices that have been the target of letters. Cross-contamination of her mail is a possible cause.

Another case involves a contract worker at a State Department mail center in suburban Virginia who has inhalation anthrax. The Centers for Disease Control and Prevention has speculated that cross-contamination of mail could not have provided enough spores to cause such a serious form of the disease and that there might be a fourth tainted letter, which sickened the State Department worker.

No other letters have been found, however. "We've seen no evidence of these other letters," Nolan said. But investigators plan to sift through impounded mail from Capitol Hill offices and from Brentwood, in some cases without waiting for it to be sanitized.

"We're all concerned that there might be other letters," said Kenneth Newman, deputy chief inspector for investigations.

The biggest mystery remains the case of Kathy T. Nguyen, 61, who died Oct. 28 in New York of inhalation anthrax. Investigators have found no evidence she came in contact with a contaminated letter.

John Nolan, deputy postmaster general, discussed the situation at a briefing at Postal Service headquarters Friday in Washington.