Even the government officials acknowledged the sorority connection is a strange one, and it’s not likely to ease concerns by Ivins’ friends and former co-workers who are skeptical about the case against him.

Ivins’ attorney, Paul F. Kemp, did not immediately respond to a message seeking comment Monday but has asserted his client’s innocence and said he would have been vindicated in court.

At least some of Ivins’ former colleagues, as well as others who want to see the FBI’s still-secret evidence, question whether he could have created the powder form of the deadly toxin without co-workers noticing.

Law enforcement officials familiar with the case said Ivins borrowed freeze-drying equipment from a bioweapons lab in 2001 that was used by scientists to convert wet anthrax spores into a powder similar to that sent to journalists and lawmakers in the weeks after the September 2001 terror attacks.

Not commonly used by researchers at the Fort Detrick, Md., lab where Ivins worked, the drying unit — known as a lyopholizer — was lent to Ivins’ unit after he went through a formal process to check it out.

While in his possession, Ivins apparently tackled a project for the Defense Advanced Research Projects Agency. Law enforcement officials, who spoke on condition of anonymity because they were not authorized to speak publicly about the inquiry, said the documents Ivins created during the check out process and in his experiments are being mined for proof that he might have used it to powderize anthrax spores.

In August 2002, investigators announced they’d found anthrax spores inside the mailbox on Nassau Street, Princeton’s main thoroughfare. FBI agents immediately began canvassing the town, showing residents a photograph of Army scientist Steven J. Hatfill, who at the time was a key “person of interest” in the case.

That theory fell flat and this June, the Justice Department exonerated Hatfill and agreed to a $5.8 million settlement with him.

In the past year, the FBI has turned its attention to Ivins, whom a therapist said had a history of homicidal and sociopathic behavior. Social worker Jean C. Duley won a protective order against Ivins on July 24 after telling a judge the scientist was a homicidal sociopath.

Duley, 45, also has a minor criminal record, according to court records. She pleaded guilty in April to driving under the influence and was fined $500 and placed on probation for nearly a year. In October 2006, she pleaded guilty to reckless driving and was fined $580. A 1992 charge of possessing drug paraphernalia was dismissed.