There was a strong sense of relief when the federal government concluded that a lone psychologically troubled government scientist mailed anthrax-laced letters in 2001, killing five people and terrorizing the nation. Now its evidence is looking increasingly shaky.

Dr. Bruce Ivins, an Army biodefense expert at Fort Detrick in Maryland, committed suicide in 2008 before the case against him could be tested in court. Independent inquiries this year have raised questions both about the genetic analyses that traced the anthrax to Dr. Ivins’s laboratory and a web of circumstantial evidence. There needs to be a new independent evaluation of the findings.

The government’s scientific case has been weakening for months. In February, the National Academy of Sciences warned that the genetic analysis “did not definitively demonstrate” that the mailed anthrax was derived from spores grown in Dr. Ivins’s laboratory. Last week, The Times reported that one of the leading anthrax authorities and two colleagues believe that distinctive chemicals in the mailed anthrax suggest it was produced by sophisticated manufacturing, which the scientists deemed far beyond Dr. Ivins’s capabilities. Although some experts think the chemicals might be meaningless contaminants, the chief of the academy panel and the leader of a pending Government Accountability Office review think the group’s assertions in a future paper need to be addressed.

As for the circumstantial evidence, an investigation by PBS Frontline, assisted by ProPublica and the McClatchy newspapers, cast doubt on two elements that prosecutors had declared important. A contention that Dr. Ivins worked extraordinarily long hours alone at night in his laboratory just before the mailings looked less suspicious after the journalists found that he regularly worked late hours in other labs and offices. And a contention that Dr. Ivins tried to mislead investigators by submitting an anthrax sample free of genetic markers looked questionable after the journalists found that he had submitted other samples that contained the markers.