The National Research Council committee held a briefing in Washington on its 170-page report, which examines the novel microbial forensic techniques used by the FBI to determine that Ivins, a researcher at Maryland's Fort Detrick, acted alone in making and sending the powdered spores.
The panel that conducted the 18-month review faulted government assertions that the mailer must have had a high level of technical skill and that the parent material of the anthrax strain used in the attacks had to have come from a flask that Ivins alone maintained.
"We find the scientific evidence to be consistent with their conclusions but not as definitive as stated," said Lehigh University President Alice P. Gast, who chaired the 16-member panel.
The FBI said in a written statement that its conclusions were based on a traditional investigation as well as scientific findings. The agency said the science provided leads but that science alone rarely solves cases.
"The FBI has long maintained that while science played a significant role, it was the totality of the investigative process that determined the outcome of the anthrax case," the agency said.
Gast declined to comment on the guilt or innocence of Ivins, who died of an apparently intentional Tylenol overdose in 2008 as the U.S. Justice Department prepared to indict him for the attacks. He had denied involvement, and his lawyer and some colleagues have maintained he was an innocent man hounded to self-destruction.
Early last year, the FBI formally closed its investigation into the anthrax letters that unnerved a nation still reeling from the 9/11 attacks, saying it had concluded that Ivins planned and executed the mailings by himself.
Five people died in October and November 2001 from anthrax inhalation or exposure linked to the letters. They were a Florida photo editor, two postal workers in Washington, a hospital employee in New York City and a 94-year-old woman in Oxford, Conn. Seventeen others were sickened.
Postal facilities, U.S. Capitol buildings and private offices were shut for inspection and cleaned by workers in hazardous materials suits from Florida to New York and elsewhere.
Investigators have acknowledged that the case against Ivins, who worked at Fort Detrick in Frederick, is circumstantial. Still, Jeff Taylor, the U.S. attorney for the District of Columbia, said in 2008 that prosecutors could prove to a jury beyond a reasonable doubt that Ivins was responsible for the attacks.
The FBI asked the congressionally chartered council to validate its use of new and emerging science in the investigation.
The panel said the science didn't support the Justice Department's statement in a 2010 report that "the anthrax mailer must have possessed significant technical skill," an assertion that narrowed who could have been responsible.
Various experts told the panel it could have taken anywhere from two days to several months to prepare the spores.
"Given uncertainty about the methods used for preparation of the spore material, the committee could reach no significant conclusions regarding the skill set of the perpetrator," the report states.
The report also challenges investigators' conclusion that the parent material of the Ames strain of anthrax spores used in the attacks came from a flask labeled RMR-1029 that was created and solely maintained by Ivins.
"The scientific link between the letter material and flask number RMR-1029 is not as conclusive as stated" by the Justice Department, the report says.
For example, the scientists found that investigators did not rigorously explore the possibility that certain genetic mutations common to the mailed anthrax and the material in Ivins' flask could have evolved separately in anthrax grown elsewhere.
The lack of certainty bolstered detractors' view that Ivins was unfairly persecuted.
"I believe that this report validates criticism that the FBI was not warranted in their aggressive pressure targeting researchers at America's finest biodefense laboratory," said Rep. Roscoe Bartlett, R-Md., whose district includes Fort Detrick.
Rep. Rush Holt, a New Jersey Democrat whose district includes the Princeton mailbox used in the attacks, has reintroduced a bill that would create a congressional commission to investigate the anthrax attacks.
"The FBI has not proven to me that this is an open-and-shut case. We still badly need a 9/11-style commission to determine how the attacks happened and whether we learned the lessons to prepare for another attack," Holt said.
The report reveals that the FBI pursued a possible al-Qaida link to the mailings by trying without success to grow anthrax from swabs and swipes taken from an unspecified overseas site at which a terrorist group's anthrax program was allegedly located. The samples had tested positive for Ames anthrax - false positives aren't unusual - but wouldn't grow spores, according to sketchy information in a newly declassified document that the FBI gave the committee in December or January.
The committee said the methods used in the inconclusive tests should be explored in more detail.
AP reporters Pete Yost and Matt Apuzzo in Washington contributed to this report.