Eight of 55 tests came back positive for anthrax contamination at a State Department remote mail facility in Virginia Tuesday, indicating a spore-laden letter remains to be found.

One State Department mail handler became ill with inhalation anthrax last month, a case that officials speculated resulted from cross-contamination with a letter mailed to Senate Majority Leader Tom Daschle.

Dr. Steven Ostroff, an anthrax expert at the Centers of Disease Control and Prevention, said that the agency believes there's a contaminated letter yet to be found in the State Department system.

State Department spokesman Richard Boucher said the results are important because they support the theory that a letter like the one sent to Tom Daschle has moved through the mail system.

Ostroff, on a conference call with reporters, said, "We have said for quite a while that one of the potential explanations for the inhalation anthrax case in that (State Department) employee was that there was an unrecognized additional letter that went through that system."

He added, "We think that based on the bulk of the evidence that's available to us the first explanation is more likely."

Boucher told reporters that eight samples out of 55 that were collected from the agency's mail facility in Sterling, Va., tested positive for anthrax. He said two of the samples were obtained from two separate mail sorters and the six others from a third sorter.

"We are now proceeding to look at all the mail that we had held up, frozen, sealed off in mailrooms in this building, in annexes and around the world," he said.

Boucher noted that the State Department closed its mail system Oct. 24 and notified posts around the world to seal and shut down pouch mail.

The State Department informed all employees of the test results at the Sterling site in a two-page "department notice."

The notice said that as soon as the U.S. Postal Service facility in the Brentwood area of Washington was discovered to have anthrax contamination, the department shut down its domestic and overseas mail systems.

This was done because of the risk of contamination at the Sterling facility, which receives most of its mail from Brentwood. The notice also recalled that all mail handlers were placed on prophylactic antibiotics.

Except for the Sterling facility, which has been shut down, "We are not at risk by working in our buildings," the notice said.

It said that existing low level contamination does not pose any risk of inhalation anthrax. "We are cleaning all our mail rooms proactively, both domestic and overseas, to be certain that we are doing what we can to best protect our employees."

Officials have located three tainted letters nationwide, one each sent to Daschle, NBC newsman Tom Brokaw and the New York Post. They also theorize that an as-yet undiscovered letter was mailed to a Florida tabloid publishing company where two employees were stricken with the disease, one fatally.

By CDC count, 17 people have been stricken with anthrax in an outbreak of bioterrorism, 10 of the inhalation form of the disease and the balance with a less serious skin type.

The Associated Press contributed to this report.