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. …

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.