The Federal Bureau of Investigation has issued a report that is supposed to clinch the case that a lone scientist mailed anthrax-laced letters in 2001, terrorizing a country already traumatized by the 9/11 attacks. The agency cites voluminous circumstantial evidence ... but its report leaves too many loose ends to be taken as a definitive verdict. The scientist - Dr. Bruce Ivins, an Army biodefense expert - killed himself in 2008 as the investigation moved ever closer to an indictment. That means the evidence and the F.B.I.'s conclusion that he was the culprit and acted alone will never be tested in court. Problematic is the investigative work that led the F.B.I. to conclude that only Dr. Ivins, among perhaps 100 scientists who had access to the same flask, could have sent the letters. The case has always been hobbled by a lack of direct evidence tying Dr. Ivins to the letters. No witnesses who saw him prepare the powdered anthrax or mail the letters. No anthrax spores in his house or car. No incriminating fingerprints, fibers or DNA. No confession to a colleague or in a suicide note, just opaque ramblings in e-mail that the F.B.I. interprets as evidence of guilt. The F.B.I. has a troubling history of building a circumstantial case against suspects who are later exonerated. We ... agree with Representative Rush Holt of New Jersey, who is calling for an independent assessment to validate the findings.