BRUCE E. IVINS has been dead for three years, but questions still abound about whether he carried out the 2001 anthrax attacks that killed five people and seriously sickened 17 others.

The FBI compiled what appeared to be an overwhelming case against the former microbiologist at Maryland’s Fort Detrick. In the weeks before anthrax-laced letters were mailed, FBI affidavits claimed, Mr. Ivins stayed late nights and weekends at the Army lab that allowed him to work undetected while cultivating the deadly spores. And the FBI provided a possible motive: An anthrax vaccine program Mr. Ivins was working on had run into trouble; an attack could give the project a boost.

But the coup de grace, according to the bureau, was provided by a cutting-edge genetic test of the anthrax spores that linked Mr. Ivins to the attack. Mr. Ivins took his own life in 2008 just as the Justice Department was preparing to charge him.

This scientific evidence came under fire in February when the National Academy of Sciences, which was commissioned by the FBI to review the case, reported that the bureau had not performed enough tests or been precise enough to conclude definitively that Mr. Ivins was the lone culprit.

Now three independent scientists have teamed up to publish a paper in the Journal of Bioterrorism and Biodefense that points to other alleged failings in the FBI’s testing procedures. Also this month, a joint investigation by PBS’s “Frontline” newsmagazine, the ProPublica online newsroom and McClatchy Newspapers raised additional questions about the accuracy of the FBI investigation. For example, the journalists revealed that Mr. Ivins handed over to investigators lab flasks that contained telltale markers found in the anthrax used in the attacks. This revelation appears to contradict FBI assertions that Mr. Ivins withheld or manipulated evidence.

Doubts about the investigation were bound to linger since Mr. Ivins killed himself before he could be tried. But the uncertainty is unacceptable. Congress should convene a panel of independent law enforcement specialists and scientists to pore over the evidence collected in the course of the FBI investigation — including classified information that was withheld from the NAS panel. Such an inquiry should attempt to get to the bottom of the 2001 attacks and assess the nation’s ability to prevent a similar attack.