Washington is preoccupied with the political decisions surrounding last year’s attack in Benghazi, but nine months later the who and why of the terrorist assault that left four Americans dead remains shrouded in mystery.

Some analysts say Ambassador J. Christopher Stevens was the target, while others believe he may have been an opportunistic way to smoke out the location of a CIA compound in the eastern Libyan city. It’s not even clear whether the attack was long premeditated or whether it was a last-minute strike organized after Stevens was seen in town the afternoon of Sept. 11.

The FBI is investigating the attack in order to fulfill President Obama’s promise to “bring to justice the killers.” But no arrests have been made and only one suspect — a Tunisian since released from custody — has been interrogated.

Attorney General Eric H. Holder Jr. dodged the issue when asked about progress last month at a congressional hearing.

“I can’t be definitive other than to say that the investigation is ongoing,” Mr. Holder told the House Judiciary Committee on May 15, though he said that the FBI has made “definitive” and “concrete” steps in the inquiry and that “we will be prepared shortly, I think, to reveal all that we have done.”