More than eight years after anthrax-laced letters killed five people and terrorized the country, the F.B.I. [has] closed its investigation, adding eerie new details to its case that the 2001 attacks were carried out by Bruce E. Ivins, an Army biodefense expert who killed himself in 2008. A 92-page report, which concludes what by many measures is the largest investigation in F.B.I. history, laid out the evidence against Dr. Ivins. The report disclosed for the first time the F.B.I.'s theory that Dr. Ivins embedded in the notes mailed with the anthrax a complex coded message, based on DNA biochemistry. Whether the voluminous documentation will convince skeptics about Dr. Ivins's guilt was uncertain. Representative Rush D. Holt, a New Jersey Democrat and a physicist who has sharply criticized the bureau's work, said the case should not have been closed. He said the F.B.I. report laid out barely a circumstantial case" that "would not, I think, stand up in court." Some of Dr. Ivins's colleagues at the United States Army Medical Research Institute of Infectious Diseases in Frederick, including several supervisors who knew him well, publicly rejected the F.B.I.'s conclusion. They said he was eccentric but incapable of such a diabolical act, and they questioned whether he could have produced the deadly powder with the equipment in his lab.