A decade after wisps of anthrax sent through the mail killed 5 people, sickened 17 others and terrorized the nation, biologists and chemists still disagree on whether federal investigators got the right man and whether the F.B.I.’s long inquiry brushed aside important clues.

Now, three scientists argue that distinctive chemicals found in the dried anthrax spores — including the unexpected presence of tin — point to a high degree of manufacturing skill, contrary to federal reassurances that the attack germs were unsophisticated. The scientists make their case in a coming issue of the Journal of Bioterrorism & Biodefense.

F.B.I. documents reviewed by The New York Times show that bureau scientists focused on tin early in their eight-year investigation, calling it an “element of interest” and a potentially critical clue to the criminal case. They later dropped their lengthy inquiry, never mentioned tin publicly and never offered any detailed account of how they thought the powder had been made.

The new paper raises the prospect — for the first time in a serious scientific forum — that the Army biodefense expert identified by the F.B.I. as the perpetrator, Bruce E. Ivins, had help in obtaining his germ weapons or conceivably was innocent of the crime.

Both the chairwoman of a National Academy of Science panel that spent a year and a half reviewing the F.B.I.’s scientific work and the director of a new review by the Government Accountability Office said the paper raised important questions that should be addressed.

Alice P. Gast, president of Lehigh University and the head of the academy panel, said that the paper “points out connections that deserve further consideration.”

Dr. Gast, a chemical engineer, said the “chemical signatures” in the mailed anthrax and their potential value to the criminal investigation had not been fully explored. “It just wasn’t pursued as vigorously as the microbiology,” she said, alluding to the analysis of micro-organisms. She also noted that the academy panel suggested a full review of classified government research on anthrax, which her panel never saw.

In interviews, the three authors said their analysis suggested that the F.B.I. might have pursued the wrong suspect and that the case should be reopened. Their position may embolden calls for a national commission to investigate the first major bioterrorist attack in American history.

But other scientists who reviewed the paper said they thought the tin might be a random contaminant, not a clue to complex processing. And the Justice Department has not altered its conclusion that the deadly letters were mailed by Dr. Ivins, an Army anthrax specialist who worked at Fort Detrick, Md., and killed himself in 2008 as prosecutors prepared to charge him.

Dean Boyd, a Justice Department spokesman, said the paper provided “no evidence whatsoever that the spores used in the mailings were produced” at a location other than Fort Detrick. He said investigators believe Dr. Ivins grew and dried the anthrax spores himself.

Story: Government lawyers backtrack on anthrax case
“Speculation regarding certain characteristics of the spores is just that — speculation,” Mr. Boyd said. “We stand by our conclusion.”

The tin is surprising because it kills micro-organisms and is used in antibacterial products. The authors of the paper say its presence in the mailed anthrax suggests that the germs, after cultivation and drying, got a specialized silicon coating, with tin as a chemical catalyst. Such coatings, known in industry as microencapsulants, are common in the manufacture of drugs and other products.

“It indicates a very special processing, and expertise,” said Martin E. Hugh-Jones, lead author of the paper and a world authority on anthrax at Louisiana State University. The deadly germs sent through the mail to news organizations and two United States senators, he added, were “far more sophisticated than needed.”

In addition to Dr. Hugh-Jones, the authors of the new paper are Barbara Hatch Rosenberg, a biologist, and Stuart Jacobsen, a chemist; both have speculated publicly about the case and criticized the F.B.I. for years.

In 2008, days after Dr. Ivins’s suicide, the bureau made public a sweeping but circumstantial case against him. Last year, the bureau formally closed the case, acknowledging that some scientific questions were unanswered but asserting that the evidence against Dr. Ivins was overwhelming.

Investigators found that the microbiologist had worked unusual late-night hours in his lab in the days before each of the two known anthrax mailings in September and October 2001; that he often mailed letters and packages under assumed names; that he had a history of homicidal threats and spoke of “Crazy Bruce” as a personality that did things he later could not remember.

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