Ivins, 62, died Tuesday in an apparent suicide in Maryland. Federal authorities had informed his lawyer that criminal charges related to the mailings would be filed.

As a co-inventor of a new anthrax vaccine, Ivins was among those in line to collect patent royalties if the product had come to market, according to an executive familiar with the matter.

The product had languished on laboratory shelves until the Sept. 11 attacks and the anthrax mailings, after which federal officials raced to stockpile vaccines and antidotes against potential biological terrorism.

A San Francisco-area biotechnology company, VaxGen, won a federal contract worth $877.5 million to provide batches of the new vaccine. The contract was the first awarded under legislation promoted by President Bush, called Project BioShield.

One executive who was familiar with the matter said that, as a condition of its purchasing the vaccine from the Army, VaxGen had agreed to share sales-related proceeds with the inventors.

"Some proportion would have been shared with the inventors," said the executive, who spoke anonymously because of contractual confidentiality. "Ivins would have stood to make tens of thousands of dollars, but not millions."

Two years after the contract was awarded to VaxGen, the pact was terminated when the company could not deliver its batches on schedule. The termination meant that VaxGen was not paid, nor were Ivins and his co-inventors.

Ivins also was listed as one of two inventors of another biodefense-related product that has won federal sponsorship.

According to their still-pending application for a U.S. patent, the inventors hoped the additive would bolster certain vaccines' capacity to prevent infections "from bioterrorism agents."

From December 2002 to December 2003, the Pentagon's Defense Advanced Research Projects Agency committed $12 million for additional testing of the experimental additive. That research money was designated for Coley Pharmaceutical Group, which was developing the additive. The company was acquired last fall by Pfizer Corp.

Samuel C. Miller, a Georgetown Law Center professor who is a patent-law expert, said that the extent to which Ivins stood to gain from the two issued patents or the one that remains pending hinges on the terms of the related contracts.

"It will depend on the business arrangements that are in place," Miller said.

On Friday, colleagues and critics of Ivins pondered the mystery within the mystery: If Ivins did it, why?

One former senior official with Ivins' employer, the U.S. Army Medical Research Institute of Infectious Diseases, whom the FBI questioned at length about Ivins, said he believed his former colleague wanted more attention -- and resources -- shifted to biological defense.

"It had to have been a motive," said the former official, who suspects that Ivins was the culprit. "I don't think he ever intended to kill anybody. He just wanted to prove 'Look, this is possible.' He probably had no clue that it would aerosolize through those envelopes and kill those postal workers."

Of the five people killed by the mailings, two worked for the U.S. Postal Service in the Washington, D.C., area; one was a photo editor in Palm Beach County, Fla.; another was a hospital supply provider in New York City; and the last known victim was a 94-year-old woman in Connecticut.

Several letters were addressed to prominent people -- two U.S. senators and NBC news anchor Tom Brokaw, for example.

For nearly 30 years, Ivins served far from the limelight, a PhD microbiologist who drew a civil servant's pay while handling some of the most deadly pathogens on Earth -- live spores of anthrax.