Dr. Stephen Ostroff, an anthrax expert with the Centers for Disease Control, said: "We don't know when and where the next event will be."

Martin E. Hugh-Jones, an anthrax expert at Louisiana State University who is aiding the federal investigation, said the mystery is likely to persist.

As they review medical journals, investigators and anthrax experts are realizing that past cases foreshadowed consequences that instead took them by surprise.

But some leading anthrax experts say that the work should have begun a year ago and that valuable information may have been lost in the meantime.

In interviews, many anthrax experts noted that victims who die are often found to have widespread damage to organs, including the brain.

"We are in the process of getting their medical records," said Dr. Bradley Perkins, an anthrax expert at the disease centers.

Another person, an anthrax expert at a government laboratory who also spoke on condition of anonymity, said that "any answer is going to be gray, and not black and white."

"Even anthrax experts have no experience doing this," a government investigator said.

"They didn't talk to me about my personal experience," said Colonel Friedlander, a physician and leading anthrax expert.

As a result of the antibioterrorism program, Dr. Koplan said, the agency now has a "world-class anthrax laboratory" and a unit of anthrax experts.