The New York Times said the findings, to be published in a coming issue of the Journal of Bioterrorism & Biodefense, suggest that Army biodefense expert Bruce E. Ivins, identified by the FBI as the suspect after he committed suicide, may have been innocent or had help in obtaining the anthrax powder.
Three scientists say in a research paper the chemicals in the anthrax, including the presence of tin, suggest extensive manufacturing skill, in contrast to federal statements that the attack germs were unsophisticated.
A Times review of FBI documents showed bureau scientists had focused on tin early in their eight-year investigation as an "element of interest" and what the newspaper called a "critical clue" in the case. But the FBI later discontinued its investigation and never provided a detailed account of how the powder was made.
The chairwoman of a National Academy of Sciences panel that spent 1 1/2 years reviewing the FBI's work and the director of a new review by the Government Accountability Office said the paper raised important questions, the Times said.
The head of the academy panel, Alice P. Gast, president of Lehigh University, said the paper "points out connections that deserve further consideration."
The three scientists said in interviews their research suggested the FBI should reopen the case.
Other scientists said the tin may have been a contaminant, not a clue to processing.
The Justice Department has not changed its conclusion that letters containing the anthrax were mailed by Ivins, an Army anthrax specialist who worked at Fort Detrick, Md. He killed himself in 2008 as prosecutors were preparing to charge him.
Justice Department spokesman Dean Boyd said the paper provided "no evidence whatsoever" that the spores were produced anywhere but at Fort Detrick and investigators believe Ivins grew and dried the anthrax spores.
"Speculation regarding certain characteristics of the spores is just that -- speculation," Boyd said. "We stand by our conclusion."
Tin kills micro-organisms and is used in antibacterial products, and its presence in the mailed anthrax suggests the germs received a silicon coating with tin as a catalyst.
"It indicates a very special processing and expertise," said Martin E. Hugh-Jones, lead author of the paper and an authority on anthrax at Louisiana State University. The mailed germs, he said, were "far more sophisticated than needed."