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.