Libyan officials said last week that four arrests had been made two days after the attack. A security official later said Libyan authorities had identified 50 people involved in the assault.
"The total number of arrests now is eight," an official from new Prime Minister Mustafa Abushagur's office said. He gave no further details.
Ambassador Christopher Stevens and three other Americans died after gunmen attacked the U.S. consulate and a safe house refuge in the eastern city on September 11.
The attackers were part of a mob blaming America for a film they said insulted the Prophet Mohammad.
(Reporting by Ali Shuaib; Writing by Marie-Louise Gumuchian; Editing by Robin Pomeroy)