Washington ? Investigators probing last fall’s anthrax attacks have no physical evidence linking Dr. Steven J. Hatfill to the crime, a federal law enforcement official said Monday, a day after Hatfill forcefully declared his innocence. Still, the FBI is unwilling to clear him.

Ten months after the attacks-by-mail, some of the nation’s top researchers are increasingly skeptical that they 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 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 potential advancement in the case. Anthrax field tests conducted on a public mailbox in the business district of Princeton, N.J., came back positive late last week, officials said. 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.

A federal law enforcement official cautioned that result came from a field test and had not been confirmed, and these tests often report anthrax where none is present. The mailbox, which is near Princeton University, has been removed for further testing, the official said.

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.