The Libyan government has advised the F.B.I. that it cannot ensure the safety of the American investigators in Benghazi. So agents have been conducting interviews from afar, relying on local Libyan authorities to help identify and arrange meetings with witnesses to the attack and working closely with the Libyans to gauge the veracity of any of those accounts.

“There’s a chance we never make it in there,” said a senior law enforcement official.

Also hampering the investigation is fear among Libyan witnesses about revealing their identities or accounts in front of Libyan guards protecting the American investigators, because the potential witnesses fear that other Libyans might reveal their participation and draw retribution from the attackers.

One person with knowledge of the inquiry said the investigators had gathered some information pointing to the involvement of members of Ansar al-Shariah, the same local extremist group that other witnesses have identified as participating in the attack. Benghazi residents and the leaders of the large militias that have constituted the city’s only police force insist that the attackers were purely local. They note that many of the brigades that have sprung up in the city have the ability to conduct such an attack on short notice and that a few homegrown groups — like Ansar al-Shariah — have the ideological disposition to do it as well.

American counterterrorism and intelligence officials say they have not found any evidence to indicate that the Qaeda affiliate in North Africa, Al Qaeda in the Islamic Maghreb, ordered or planned the attack.

“Those individuals — whoever they may be — who took part in the attack all swim in the same, relatively small, extremist pond,” said one American official. “So there could be a number of individual or ad hoc ties with A.Q.I.M. or other extremist groups. These connections alone don’t mean A.Q.I.M. was behind or planned the attack.”