WASHINGTON — One of the biggest mysteries surrounding the anthrax scare of 2001 — why the anthrax-laced letters were dropped off at a mailbox in New Jersey — may be connected to a sorority chapter at Princeton University.

Bruce Ivins’ decades-long obsession with the Kappa Kappa Gamma sorority could link the former Army biowarfare scientist to the four anthrax-laced letters, authorities said Monday.

Still, authorities acknowledge they cannot place Ivins in Princeton the day the anthrax was mailed. And the curious explanation connecting the scientist and a sorority is unlikely to satisfy his friends and former co-workers who question what motive the married father of two might have had for unleashing the attack.

Ivins, 62, killed himself last week as the Justice Department prepared to indict him on capital murder charges for the deaths of five people who were poisoned by the anthrax in the weeks following 9/11. His attorney maintains he would have been proven innocent were he still alive.

The mailbox just off the campus of Princeton University where the letters were mailed sits about 100 yards away from where the college’s Kappa Kappa Gamma chapter stores its rush materials, initiation robes and other property. Sorority members do not live there, and the Kappa chapter at Princeton does not provide a house for the women.

Multiple U.S. officials told The Associated Press that Ivins was obsessed with Kappa Kappa Gamma, going back as far as his own college days at the University of Cincinnati when he apparently was rebuffed by a woman in the sorority. The officials all spoke on condition of anonymity because they were not authorized to discuss the case publicly.

There is nothing to indicate Ivins was focused on any one sorority member or other Princeton student, the officials said. Instead, officials said, Ivins’ e-mails and other documents detail his long-standing fixation on the organization.

An adviser to the Kappa Kappa Gamma chapter at Princeton, Katherine Breckinridge Graham, said Monday she was interviewed by FBI agents “over the last couple of years” about the case. She said she could not provide any details about the interview because she signed an FBI nondisclosure form.

However, Graham said there was nothing to indicate that any of the sorority members had anything to do with Ivins.

“Nothing odd went on,” said Graham, an attorney and Kappa alumna.

Kappa Kappa Gamma executive director Lauren Paitson, reached at the sorority’s headquarters in Columbus, Ohio, initially told an AP reporter Monday afternoon she would provide a comment shortly. She did not answer subsequent phone messages or e-mails seeking that response.

Had he lived, authorities had planned to argue that Ivins could have made the seven-hour round trip to Princeton from the Fort Detrick lab in Frederick, Md., after work. One official said investigators were working off the theory that Ivins chose to mail the letters from outside the sorority’s Princeton chapter to confuse the government if he ever were to emerge as a suspect in the case.

Kappa Kappa Gamma also has chapters at colleges in Maryland, Pennsylvania and Washington.

Princeton University referred questions about Ivins to the FBI. The university does not formally recognize sororities and fraternities, but chapters operate off campus. Local police in both Princeton Borough and Princeton Township said Ivins’ name did not turn up on any incident reports or restraining orders.

Details about Ivins’ alleged obsession with the sorority will be spelled out in court documents that could be made public as early as Tuesday. The Justice Department is expected to decide soon whether to end the “Amerithrax” investigation by concluding Ivins acted alone in carrying out the attacks that killed five and sickened 17.

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.