“What we have seen over the past few days has been a mix of improper disclosures of partial information mixed with inaccurate information and then drawn into unfounded conclusions,” said Brian Roehrkasse, a spokesman for the Justice Department. “None of that serves the victims, their families or the public. Likewise, we will not discuss reports or details on the timing of briefings to the public or victims and their families. We will provide such details to the press at the appropriate time, and not before.”

Dr. Ivins, who worked on anthrax vaccines for 18 years at the military research facility at Fort Detrick in Frederick, Md., took a fatal overdose of Tylenol and codeine last week at his home near the base. He had been under investigation for more than a year in the anthrax killings and had recently been told that the Justice Department was on the verge of seeking an indictment against him on capital murder charges.

Federal officials, speaking on the condition of anonymity because the case remains under seal, said they were confident that Dr. Ivins was the anthrax killer and that he had acted alone. They said that new scientific methods had allowed the F.B.I. to trace the genetic makeup of the anthrax sample to Dr. Ivins and a pool of about 10 other people who had access to a particular supply of anthrax at Fort Detrick. Other evidence, much of it still undisclosed, amounted to a strong circumstantial case against him, the officials said. Perhaps the most provocative piece of evidence to emerge publicly is the testimony of a therapist who treated Dr. Ivins in recent months and described him as homicidal.

But Dr. Ivins’s lawyer has asserted his innocence, and a number of colleagues at Fort Detrick have defended him, saying that his recent mental state and his suicide were the result of many months of near-constant surveillance and scrutiny by the F.B.I., not a reflection of his guilt.

Some government officials have also questioned the strength of the bureau’s case and said they were eager to see the grand jury documents.

One Congressional official briefed on the case said he was not persuaded that the F.B.I. had made a credible case in singling out Dr. Ivins in the group of people at Fort Detrick who had access to anthrax samples linked to the 2001 attacks.

The F.B.I. may be able to point to odd behavior on the part of Dr. Ivins, said the official, who spoke on condition of anonymity because the case is under seal. But he said the attention the bureau focused on Dr. Ivins was reminiscent of a past misstep: “It looks like what they did to Hatfill. Ivins was the weirdest one.”