WASHINGTON - Federal officials said yesterday they believe an anthrax-filled letter that has yet to be discovered sickened a State Department mail handler with the most serious form of the disease, a theory bolstered by the discovery of anthrax in eight places in the building where he worked.

The State Department said it would begin hunting through three weeks' worth of unopened mail, searching for a letter that could advance the anthrax investigation.

"We have to assume that, one, there is a contaminated letter of some kind in our system, and second of all, that we will eventually find it in one of these mail rooms or pouch bags," said State Department spokesman Richard Boucher.

More than two weeks ago, Dr. Jeffrey Koplan, director of the Centers for Disease Control and Prevention, said it was a virtual certainty that another letter was lurking, undiscovered. On Tuesday, the State Department said it didn't begin looking sooner because the mail facility was "a crime scene" and officials wanted to test it for anthrax first.

It wasn't clear why looking through the mail would hamper the testing, and CDC officials couldn't immediately say whether they had recommended the delay. For its part, the FBI said it doubts a letter will be found even once they start looking.

Also yesterday, the last of six people to survive inhalation anthrax came home after 25 days in a suburban Washington hospital. Leroy Richmond, a postal worker at the city's contaminated central facility, said he was grateful to doctors who began treating him for anthrax even before it was confirmed. "Timing was crucial," he said.

Four others have died from the disease, including two area postal workers remembered yesterday at a Postal Service memorial.

No new cases of anthrax infection have been reported for more than two weeks, though traces of the bacteria have continued to turn up. Eight mailrooms at Howard University in Washington were closed after spores were found, following a weekend when anthrax was detected in 11 congressional offices. The amounts were so small that none of these offices face a hazard, health officials say.

There was more concern after anthrax was detected in eight spots in the State Department's Sterling, Va., mail facility. Six of them were on a single automated mail sorter, suggesting that a letter containing a substantial amount of anthrax passed through it, Boucher said yesterday.

The only other possibility is that State Department mail touched an anthrax-filled letter sent to Senate Majority Leader Tom Daschle and became contaminated. But health officials have said it's highly unlikely that cross-contamination would involve enough airborne spores to give a mail handler inhalation anthrax.

Now, with a significant dose of anthrax found on the State Department mail machines, it seems certain there's another letter, Boucher said. He compared this undiscovered letter to the Daschle letter, which contained two grams of highly concentrated anthrax. Test results, he said, "support the theory that there is a letter like the one sent to Senator Daschle that has moved through our mail system."

As a result, he said, the department was now planning to examine undelivered mail in Washington and in embassies around the world that passed through the Sterling facility and has been held since Oct. 24.

Boucher added that if an anthrax-tainted letter had slipped through before the mail was shut down, someone would have discovered it by now.

It was unclear why the State Department was just now preparing to examine this unopened mail.

Boucher said the department is working with the CDC, but CDC officials said more than two weeks ago that they believed there was another letter, not yet discovered.

The environmental testing simply bolsters their theory, CDC officials said yesterday.

"Our hypothesis ... from the first point that this case was identified was that there's got to be some probability that there's another letter that's gone through here," Koplan said.

At the FBI, a spokesman said investigators had not tested the mail earlier because FBI officials believe that, if there was another letter, it had already been delivered.

But Boucher said that's not likely. If an anthrax letter had been delivered, he noted, someone presumably would have opened it and noticed the powder or gotten sick.

"We have to assume that we stopped it, we stuck it in our system where all the mail is sealed," he said. "We have to presume that we will be able to find whatever it is as we go through the mail in our system."