The FBI is not ready to clear Dr. Steven J. Hatfill in last fall's anthrax attacks even though investors have no physical evidence linking him to the crime, says a federal law enforcement official.

The official spoke on condition of anonymity Monday — the day after Hatfill forcefully declared his innocence.

With the trail growing colder 10 months after the attacks, some top researchers are increasingly skeptical investigators will ever find evidence linking anyone to the crime.

Hatfill, a biowarfare expert whose name surfaced more than a month ago, has attracted attention because of intriguing circumstances about his past. Among them: a novel he wrote about a bioterror attack. Hatfill copyrighted the book "Emergence" in 1998 with co-author Roger Akers, who indicated in an interview Monday that the book dealt with an anthrax attack on Congress.

Investigators have searched Hatfill's Frederick, Md., apartment twice, including testing for anthrax residue, as well as his car, a storage unit in Florida and his girlfriend's home. They have seized his computer and bags of personal items he had thrown away in preparation for moving. But they have found nothing to link Hatfill — or anyone else — to the tainted letters that killed five people last fall, the official said.

Law enforcement officials have described Hatfill, 48, as a "person of interest," not a criminal suspect, and said he is one of about 30 people being scrutinized.

There was one advancement in the case: Laboratory tests came back positive on a public mailbox last week in the business district of Princeton, N.J., said Dr. Clifton R. Lacy, commissioner of the New Jersey Department of Health and Senior Services.

Although one U.S. law enforcement official earlier said the results came from less-reliable field testing, Lacy said Monday the positive result came from full lab tests.

Of the 561 mailboxes so far tested for anthrax, this is the only one to register anthrax, said New Jersey Gov. James E. McGreevey. The deadly letters were postmarked in New Jersey, though investigators have not pinpointed the mailbox where they were dropped.

Scientists say they are not surprised that the investigation remains stalled. Too many months have passed, they say, and it would be easy for a skilled scientist to have disposed of any evidence that remained.

Over-the-counter bleach will destroy any trace of anthrax, noted Philip Hanna, a microbiologist at the University of Michigan Medical School. "Chances of finding something get more and more remote."

Experts said a skilled scientist could have manufactured the quantity and quality of last year's anthrax attacks in a remote location with specialized equipment, especially if the scientist wasn't unduly concerned about risking exposure to himself.

"My best technical guess is that somebody could have grown a gram or so with equipment that fit on the top of your desk," said Jay C. Davis, former director of the Defense Threat Reduction Agency, part of the Defense Department.

Richard Ebright, a chemistry professor at Rutgers University, estimated that the equipment needed could be purchased for less than $50,000. And Davis said it would be relatively simple for the suspect in the fall's anthrax attacks to cover his tracks.

The envelopes sent to Capitol Hill containing the anthrax contained no fingerprints, suggesting the use of gloves; and the stamps were the self-stick kind, removing the possibility of using microscopic traces of saliva on a licked stamp to identify a suspect.

On Sunday, Hatfill said his efforts to cooperate with the investigation have been met with news leaks and character assassination.

The law enforcement official said Hatfill has been straightforward answering questions, but investigators are unwilling to declare him cleared of any suspicion because of intriguing circumstances about his past. In addition to finding on his computer the draft of a novel about a bioterrorism attack, investigators found:

• The anthrax letters contained a return address of a nonexistent Greendale School in New Jersey. Hatfill once lived in Harare, Zimbabwe -- near the suburb of Greendale, where there is a school named for Courtney Selous. Selous is namesake of the Selous Scouts, who fought for white rule in what was then called Rhodesia; Hatfill has said he fought with the Selous Scouts.

• In 1999, while working for a defense contractor, Hatfill commissioned a report looking at how anthrax might be sent through the mail. That report suggested there would be about 2.5 grams of anthrax in an envelope -- and that's what was in last fall's mailings.