Mr. Miller said the bureau ultimately planned to release much more information from its investigative files, including notes of F.B.I. interviews with Dr. Ivins and other suspects and witnesses and surveillance logs detailing his movements and actions. But those disclosures, requiring a detailed review to remove private and classified information, are likely to be months away.

Mr. Mueller, the F.B.I. director, is likely to face tough questions at next week’s scheduled oversight hearings, not just about the case against Dr. Ivins but about the prolonged pursuit of Dr. Hatfill. Senator Charles E. Grassley, an Iowa Republican and frequent critic of the bureau, said he was frustrated by the delay in closing the case and answering questions.

“If the case is solved, why isn’t it solved?” Mr. Grassley asked. “It’s all very suspicious, and you wonder whether or not the F.B.I. doesn’t have something to cover up and that they don’t want to come clean.”

Investigators have not reviewed three boxes of papers left by Dr. Ivins marked for the attention of his lawyer, Paul F. Kemp, because the records must first be reviewed to see whether they should be kept confidential under attorney-client privilege, Mr. Kemp said. A government lawyer not involved in the investigation will soon review the papers with Mr. Kemp, who said some might be given to investigators or made public.

What is clear is that the disclosures have not closed the matter.

“They took their shot,” said Representative Rush D. Holt, a Democrat who holds a doctorate in physics and has followed the case closely because the letters were mailed in his New Jersey district. “They hoped and maybe believed that the case they laid out would persuade everyone. I think they’re probably surprised by the level of skepticism.”

Many scientists who have tracked the case, too, have found the evidence less than decisive.

“For a lot of the scientific community, the word would be agnostic,” said Dr. Thomas V. Inglesby, an expert on bioterrorism at the Center for Biosecurity of the University of Pittsburgh Medical Center. “They still don’t feel they have enough information to judge whether the case has been solved.”

Mr. Holt and Dr. Inglesby were among a number of outsiders who said that only an independent review of the investigation and the evidence against Dr. Ivins  either by Congress or a commission like the one that investigated the Sept. 11 attacks  could give the public confidence that the case was over.