A congressional inquiry into the F.B.I.’s scientific work on the anthrax mailings of 2001 has identified major gaps in genetic evidence that purportedly links the germs to Bruce E. Ivins, the Army microbiologist blamed for attacks that killed five people, sickened 17 others and shook the nation.

The Government Accountability Office study, requested in 2010 and made public on Friday, echoes earlier criticism from the National Academy of Sciences. In 2011, its expert panel found that the bureau’s analysis of the genetic evidence “did not definitively demonstrate” a firm link between the mailed anthrax spores and a sample taken from Dr. Ivins’s laboratory at Fort Detrick in Maryland, and more generally was “not as conclusive” as the bureau had asserted.

The G.A.O. had better access to F.B.I. records and deepened the genetic critique, finding that the bureau’s investigation “lacked several important characteristics” that could have strengthened its case. “A key scientific gap,” the 77-page report said, was the bureau’s failure to investigate whether samples of anthrax spores could naturally mutate enough to obscure their putative links to Dr. Ivins.

In 2008, shortly after he killed himself, the bureau laid out a sweeping but circumstantial case against Dr. Ivins, an Army microbiologist, saying he had acted alone in conducting the nation’s first major bioterrorist attack. It called the case Amerithrax and said that unique mutations in the anthrax spores had helped put Dr. Ivins under the spotlight.