An independent panel of scientists has determined that the FBI did not have enough scientific evidence to produce a conviction in the case of the 2001 anthrax attacks that killed five people.

The National Academies of Sciences released a review Tuesday of the science used in the investigation. The $1.1 million report, which was commissioned by the FBI, concluded that the man accused in the case, Bruce Ivins, could have carried out the attacks, but the science alone did not prove it.

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In October and September of 2001, letters containing anthrax killed five people and infected 17 others. Recipients included NBC News, The New York Post, Sen. Tom Daschle (D-SD) and Sen. Patrick Leahy (D-VT).

Even after over 600,000 investigator work hours spent by the FBI’s “Amerithrax Task Force,” the case against Ivins was largely circumstantial.

Ivins killed himself in 2008 just as the government was prepared to indict him. The Justice Department closed the case last year, concluding Ivins had acted alone in stealing the spores from the government lab where he worked.

The report released Tuesday questioned the link between a flask of anthrax found in Ivins’ office and the letters.

“The scientific link between the letter material and flask number RMR-1029 is not as conclusive as stated in the DOJ Investigative Summary,” the report said.

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The panel added that another explanation for the link “was not rigorously explored” by the FBI.

“This shows what we’ve been saying all along: that it was all supposition based on conjecture based on guesswork, without any proof whatsoever,” Paul Kemp, a lawyer who represented Ivins, told The Washington Post.

“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 Justice Department and the FBI said in a joint statement. “Although there have been great strides in forensic science over the years, rarely does science alone solve an investigation.”

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In a September 2007 e-mail to himself, Ivins said he knew of the identity of the anthrax killer. Before his death in 2008, he told friends that government agents had hounded him and his family. These details have given rise to a wide variety of conspiracy theories about the case ever since.