WASHINGTON -- Escorted by several dozen Special Operations forces, F.B.I. agents on Thursday entered the ruins of the American diplomatic compound in Benghazi, Libya, as part of their investigation into the killings there of ambassador J. Christopher Stevens and three other Americans.
Security fears had kept the F.B.I. agents from traveling the 400 miles from the American Embassy in Tripoli, the Libyan capital, to collect evidence at a crime scene that was trampled, looted and badly burned by militants more than three weeks ago. Administration officials said Thursday the delay was caused in part by the Libyan government, which they described as slow in granting approval for the mission.
The officials said the agents flew from Tripoli in a C-130 military transport plane and were then driven to the compound in armored cars. The officials did not say how many F.B.I. agents were involved or precisely how long they were on the ground. The Pentagon press secretary, George Little, would only say at a briefing that the agents and their military escorts were in Benghazi "for a number of hours" before returning to Tripoli.
The agents were specialists in evidence collection, according to law enforcement officials, and were there to sift through the wreckage and to determine in better detail how the attack unfolded. It is unclear how much can still be gleaned from the site, which a senior American law enforcement official has described as so badly "degraded" that linking evidence to the attackers will be difficult at best.
Already looters, curiosity seekers and reporters have been through the site, which is only protected by two private security guards hired by the compound's Libyan owner, The Washington Post reported Thursday. On Wednesday, a Post reporter at the site discovered loosely secured sensitive documents about American operations in Libya; the newspaper sent copies of some of them to the State Department. Last month CNN discovered Mr. Stevens's diary in the wreckage.
It is unclear if the F.B.I. investigators plan to return to the site, but Mr. Little hinted that they might. He offered few details about the military escort operation, adding, "We may need to replicate it in the future, and I wouldn't want to tip off the wrong people."
It appears that the F.B.I. spent little or no time interviewing residents in Benghazi. Typically they would spend weeks, rather than hours, at a crime scene as important to national security as this site. The F.B.I., which always investigates the deaths of American overseas under suspicious circumstances, has agents from its national security division and New York field office in Libya. They have been operating largely out of the American Embassy in Tripoli, now guarded by a force of 50 elite Marines trained to protect American diplomatic posts in crisis.
But even in Tripoli the investigation has been hobbled by the tenuous security in Libya after the overthrow of Col. Muammar el-Qaddafi. Late last month investigators were so fearful about the risks of taking some potential Libyan witnesses into the American Embassy that they resorted to questioning people in cars outside the embassy.
The agents are also operating without any help on the ground from the C.I.A., which had about a dozen intelligence operatives and contractors in Benghazi until the attacks, conducting surveillance and collecting information on militant groups in the city. They were among more than two dozen American personnel evacuated from Benghazi after the attack.
American counterterrorism officials and Benghazi residents are now focused on a local militant group, Ansar al-Shariah, as the main force behind the attack, which occurred on the 11th anniversary of the Sept. 11 attacks on the World Trade Center and the Pentagon.
President Obama has vowed to bring the killers to justice, and the United States is now laying the groundwork for possible operations to kill or capture militants implicated in the attack. The options could include drone strikes, Special Operations raids and joint missions with the Libyan authorities. But the Libyan government opposes any unilateral American military operation in Libya against the attackers, and administration officials say no decisions have been made about attacking any potential targets.
Correction: October 5, 2012, Friday
This article has been revised to reflect the following correction: An earlier version of this article referred imprecisely to what The Washington Post did with documents that were found by a Post reporter at the site. The Post provided copies of some of the documents to the State Department. It did not turn over any of the original documents to the State Department.
This article originally appeared in The New York Times.