The F.B.I. seems convinced that it has finally solved the long-festering case of who mailed the anthrax letters that killed five people in 2001. Yet its description of the evidence pointing to a mentally disturbed Army bioweapons expert as the sole culprit leaves us uncertain about whether investigators have pulled off a brilliant coup after a bumbling start, or are prematurely declaring victory, despite a lack of hard, incontrovertible proof.

    Federal agents relied on sophisticated scientific tests and laborious investigative work to conclude that only Dr. Bruce Ivins, who killed himself last week, could have made and mailed the anthrax used in the letters. They say that recently developed tests enabled them to identify telltale genetic mutations in the anthrax and to show that it came from a flask kept by Dr. Ivins at the Army laboratories at Fort Detrick in Frederick, Md.

    More than 100 people might have had access to the deadly substance, but over a four-year period, investigators gradually eliminated suspects until only Dr. Ivins was left.

    None of the investigators' major assertions, however, have been tested in cross-examination or evaluated by outside specialists. It is imperative that federal officials make public all of their data so independent experts can judge whether the mailed anthrax was indeed identical to Dr. Ivins's supply and only that supply.

    It is also critical for officials to explain more fully how they eliminated the many other people with access to the material.

    The investigators came up with lots of circumstantial evidence to bolster their case. The envelopes used in the mailings had print defects that indicate that they could only have been bought at a small number of post offices, including one where Dr. Ivins had a mailbox under an assumed name. Dr. Ivins had obtained equipment that could turn his wet anthrax into the dry spores that were mailed.

    In the days before the mailings, he worked long hours alone at night and on weekends, outside his usual pattern. E-mail messages indicate that he was mentally disturbed and worried about an anthrax vaccine project that was failing, a possible motive for mailings to generate public fear about anthrax. There is also evidence that he had a history of driving to other locations to mail packages anonymously.

    But there is no direct evidence of his guilt. No witness who saw him pouring powdered anthrax into envelopes. No anthrax spores in his house or cars. No confession to a colleague or in a suicide note. No physical evidence tying him to the site in Princeton, N.J., from which the letters are believed to have been mailed.

    Because Dr. Ivins killed himself before he could be indicted, there will be no opportunity for an adversarial testing of the F.B.I.'s conclusions. The bureau, unfortunately, has a history of building circumstantial cases that seem compelling at first but ultimately fall apart. Congress will need to probe the adequacy of this investigation - and to insist that federal officials release as much evidence as possible, so the public can be assured they really did get the right person this time.

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