Security Fears Hobble Inquiry of Libya Attack Mohammad Hannon/Associated Press, for The New York Times The F.B.I. has not been able to reach the American Mission in Benghazi, shown on Sept. BENGHAZI, Libya - Sixteen days after the death of four Americans in an attack on a United States diplomatic mission here, fears about the near-total lack of security have kept F.B.I. agents from visiting the scene of the killings and forced them to try to piece together the complicated crime from Tripoli, more than 400 miles away. Investigators are so worried about the tenuous security, people involved in the investigation say, that they have been unwilling to risk taking some potential Libyan witnesses into the American Embassy in Tripoli. Instead, the investigators have resorted to the awkward solution of questioning some witnesses in cars outside the embassy, which is operating under emergency staffing and was evacuated of even more diplomats on Thursday because of a heightened security alert. "It's a cavalcade of obstacles right now," said a senior American law enforcement official who is receiving regular updates on the Benghazi investigation and who described the crime scene, which has been trampled on, looted and burned, as so badly "degraded" that even once F.B.I. agents do eventually gain access, "it'll be very difficult to see what evidence can be attributed to the bad guys." Piecing together exactly how Ambassador J. Christopher Stevens and three other Americans died here would be difficult even under the best of conditions. But the volatile security situation in post-Qaddafi Libya has added to the challenge of determining whether it was purely a local group of extremists who initiated the fatal assault or whether the attackers had ties to international terrorist groups, as Secretary of State Hillary Rodham Clinton suggested Wednesday. 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. But the investigators are casting a wide net. To determine whether there was participation by an international element, intelligence analysts are poring over cellphone conversations intercepted before and after the attacks, as well as informant reports, witness accounts and satellite imagery. When asked which group or groups may have been behind the violence, Matthew G. Olsen, director of the National Counterterrorism Center, told senators in Washington last week, "The picture that is emerging is one where a number of different individuals were involved, so it's not necessarily an either/or proposition." David D. Kirkpatrick reported from Benghazi and Tripoli, Libya, and Eric Schmitt and Michael S. Schmidt from Washington. Adam Nossiter contributed reporting from Bissau, Guinea-Bissau, and Steven Lee Myers from New York.