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.

That evidence includes tracing the prestamped envelopes used in the attacks to stock sold in three Maryland post offices, including one in Frederick, frequented by Dr. Ivins, who had long rented a post office box there under an assumed name, the source said. The evidence also includes records of the scientist’s extensive after-hours use of his lab at the United States Army Medical Research Institute of Infectious Diseases around the time the letters were mailed, the source said.