NEW YORK -- A decade after wisps of anthrax sent through the mail killed five people, sickened 17 others, and terrorized the nation, biologists and chemists still disagree on whether federal investigators got the right man and whether the FBI's long inquiry brushed aside important clues.
Three scientists now 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 the attack germs were unsophisticated.
They make their case in a coming issue of the Journal of Bioterrorism & Biodefense.
FBI documents 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 case.
They later dropped their inquiry, formally closing the case last year, never mentioned the inquiry publicly, and offered no 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 FBI as the perpetrator, Bruce Ivins, had help in obtaining his germ weapons or conceivably was innocent of the crime.
The chairman of a National Academy of Science panel that spent a year and a half reviewing the FBI's scientific work and the head of a new review by the Government Accountability Office said the paper raised important questions that should be addressed.
Alice Gast, president of Lehigh University and head of the academy panel, said the paper "points out connections that deserve further consideration." Ms. Gast, a chemical engineer, said the "chemical signatures" in the mailed anthrax and their potential value to the criminal probe had not been fully explored.
"It just wasn't pursued as vigorously as the microbiology," she said, alluding to how germs are grown and the analysis of their genetic makeup. She also noted 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 the FBI might have pursued the wrong suspect and the case should be reopened.
Their position may embolden calls for a national commission to investigate the first major bioterrorist attack in U.S. history.
But other scientists who reviewed the paper said they thought the tin might be random contaminants, not clues to complex processing.
The Justice Department has not altered its conclusion that the deadly letters were mailed by Mr. 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 that the spores used in the mailings were produced" at a location other than Fort Detrick. He said investigators believe Mr. Ivins grew and dried the anthrax spores himself.
"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 microorganisms 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 the industry as microencapsulants, are common in the manufacture of drugs and other products.
"It indicates a very special processing, and expertise," said Martin Hugh-Jones, lead author of the paper and a world authority on anthrax at Louisiana State University. The deadly germs sent by mail to news organizations and two U.S. senators, he added, were "far more sophisticated than needed."
No evidence directly tied Mr. Ivins to the anthrax mailings in September and October, 2001. Some of the scientist's former colleagues have argued that he could not have made the anthrax and that investigators hounded a troubled man to death.
They noted the FBI pursued several other suspects, most notoriously another former Army scientist, Steven J. Hatfill, whom the bureau eventually exonerated and paid a $4.6 million legal settlement.
In a February report, the National Academy of Sciences panel criticized some of the FBI's scientific work, saying the genetic link between the attack anthrax and a supply in Mr. Ivins' lab was "not as conclusive" as the bureau said.
If the authors of the new paper are correct about the silicon-tin coating, it appears likely Mr. Ivins could not have made the anthrax powder alone with the equipment he possessed, as the FBI says. That would mean he got the powder from elsewhere or he was not the perpetrator.
The Government Accountability Office, the investigative arm of Congress, is conducting its own review of the anthrax evidence.