WASHINGTON —More than nine years following the mailing of anthrax letters that killed five people and sickened 17, a National Research Council committee reported Tuesday that the F.B.I. overstated the connection between the anthrax in the letters and spores from a laboratory used by the government scientist accused in the deadly mailings.

Last year, the F.B.I. said the anthrax used in the mailings was identical to strains kept in a laboratory used by Bruce E. Ivins, a scientist at the U.S. Army Medical Research Institute of Infectious Diseases (USAMRIID ) at Fort Detrick, Md. It said Ivins, who committed suicide in 2008 with a cocktail of Tylenol and codeine before he could be charged, was solely responsible for the attacks.

The panel said the scientific evidence was consistent with the F.B.I.’s conclusions but not definitive.

“The F.B.I. created a [catalog of anthrax] samples and performed experiments to show the relationship between the letter materials and the [test tube at USAMRID.] It is not as conclusive as [was] stated,” said Alice P. Gast, committee chair and president of Lehigh University in Bethlehem, Pa.

The report says that “it is not possible to reach a definitive conclusion about the origins of the [anthrax] in letters mailed to New York City and Washington, D.C., based solely on the available scientific evidence.”

In a written response to the report, the F.B.I. said that they have “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.”

David A. Relman, vice committee chair and professor of Medicine, Microbiology and Immunology at Stanford University, said that even today government investigators face obstacles in conducting a scientific investigation and that the F.B.I. was correct in reaching out to the scientific community to help.

“Today, and even at that time, one was limited in what was available for testing and may have been limited in the ways it had been collected for testing… So to look at today for what might have been a different answer is…difficult,” Relman said.

Friends and colleagues of Ivins maintained his innocence and argued that his anxious behavior and later suicide were caused by stress from the F.B.I. investigation.

Following the 2001 mailings, authorities identified several ‘persons of interest,’ one of them being Steven J. Hatfill, who was named on national television by Attorney General John Ashcroft. Hatfill sued the federal government and in 2008 the Justice Department agreed to pay Hatfill $4.6 million to settle the suit.