The evidence amassed by F.B.I. investigators against Dr. Bruce E. Ivins, the Army scientist who killed himself last week after learning that he was likely to be charged in the anthrax letter attacks of 2001, was largely circumstantial, and a grand jury in Washington was planning to hear several more weeks of testimony before issuing an indictment, a person who has been briefed on the investigation said on Sunday.

While genetic analysis had linked the anthrax letters to a supply of the deadly bacterium in Dr. Ivins’s laboratory at Fort Detrick, Md., at least 10 people had access to the flask containing that anthrax, said the source, who spoke on condition of anonymity because he was not authorized to discuss the investigation publicly.

Agents from the Federal Bureau of Investigation also have no evidence proving that Dr. Ivins visited New Jersey on the dates in September and October 2001 when investigators believe the letters were sent from a Princeton mailbox, the source said.

The source acknowledged that there might be some elements of the evidence of which he was unaware. And while he characterized what he did know about as “damning,” he said that instead of irrefutable proof, investigators had an array of indirect evidence that they argue strongly implicates Dr. Ivins in the attacks, which killed 5 people and sickened 17 others.