Senior law enforcement officials said the investigators were in Tripoli awaiting approval to travel to Benghazi, where Ambassador Christopher Stevens and three other diplomatic staff employees were killed Sept. 11 at the U.S. Consulate in the eastern Libyan city, CNN reported.
In addition, the crime scene has not been secured, prompting concerns about how useful any information gathered will be, officials said.
FBI agents made a request through the U.S. State Department for the crime scene to be secured but nothing has happened, CNN reported.
Last Thursday, U.S. Secretary of State Hillary Clinton told reporters an FBI team had arrived in Libya earlier in the week.
FBI investigators interviewed members of the U.S. State Department and other U.S. government personnel in Libya at the time of the attack, but the FBI's request to directly question individuals in Libyan custody was denied, CNN said.
Libyan officials have said they detained dozens of people since the attack, which officials said occurred during a protest about a U.S.-made film that debased the Muslim Prophet Muhammad.
A law enforcement source told CNN investigators have known since the beginning that Stevens and the others were killed in a terrorist attack. However, officials offered conflicting assessments of what happened, with the initial accounts alleging protesters angry about the film fueled the deadly violence, which has drawn sharp criticism from Republican lawmakers.