Now, reports CBS News Correspondent Jim Stewart, they've begun watching other suspects, but the result is the same: no hard evidence, no hot tips and no arrests.
FBI Assistant Director Van Harp heads up the investigation.
"During the course of this investigation there have been a couple of suspects that have come to the top and we've taken a look at them, and we have conducted some surveillance. We don't have a target on anyone, we have not identified anyone as 'the person'," he said.
However, the FBI - renown for its behavioral profiles of criminal suspects - does have some clues about the suspected anthrax-mailer.
The agency believes it has narrowed the world of suspects down to a dozen or more, but getting beyond that has proven torturous.
But there has been progress and much of that comes from the anthrax itself. The poisonous spores may even contain a roadmap to their maker.
"We expect in the next 30, 90 days, maybe even six months, but in that timeframe, some very specific concrete conclusions," said Harp.
And after searching thousands of copiers in New Jersey where the letters were mailed, agents believe they've found the very one the killer used to make his duplicates by isolating the tiny scratches and smears unique to each machine. They've staked out mail drops, reviewed school records and even checked on the deaths of a few suspicious men on the off chance the suspect was killed by his own spores.
Still, Professor Barbara Rosenberg says it all keeps coming back to one small set of scientists.
"The strain that was used and the method by which the anthrax was weaponized, the way it was treated, all indicate that the letters were sent by a person who was an insider in the bio-defense program," she said.
Someone, in short, who knows he's under suspicion and smart enough not to show it.
Harp also said the FBI believes that, because the mailed anthrax was of the so-called "Ames strain" of Bacillus anthracis, the suspect probably has or had legitimate access to biological agents in a laboratory. Harp also described the suspect as "stand-offish" and preferring to work alone rather than in groups.
"It is possible this person used off-hours in a laboratory or may have even established an improvised or concealed facility comprised of sufficient equipment to produce the anthrax," Harp said.