Rights groups have raised concerns about the Libyan legal system, which is in the early days of being rebuilt following the fall of the Gadhafi regime last year.
At the same time, the dominance of militia groups, "which in most towns and cities are stronger than the army and police, has complicated the rebuilding of Libya's justice system," the report says.
Amnesty International has similarly warned of serious problems with Libya's legal system. Its researchers also "found that hundreds of armed militias are acting above the law."
Questions swirl around the attack
There are numerous questions about what happened at the consulate where protesters had gathered to demonstrate against the film "Innocence of Muslims," which reportedly was made in California by a filmmaker whose identity is unclear.
Chief among the questions is what happened to Stevens, who went missing during the attack.
What is known is that during the attack, a rocket-propelled grenade set the consulate on fire, and American and Libyan security personnel tried to fight the attackers and the fire.
As the fire spread, three people -- Stevens, Foreign Service information management officer Sean Smith, and a U.S. regional security officer -- were inside a safe room, said senior State Department officials, who spoke on condition of anonymity as a matter of practice during a briefing with reporters.
Smith was later found dead, apparently of smoke inhalation, officials said.
The State Department has not released details about how Stevens died, though numerous media reports have said the ambassador was taken from the consulate to the Benghazi Medical Center by locals.
He arrived at the hospital, according to the reports, unresponsive and covered in soot from the fire.
A doctor was unable to revive him and declared him dead, the reports said.
Stevens' body was turned over to consulate personnel as they were evacuated from Benghazi.
Also killed in the attack on the consulate were security personnel Glen Doherty and Tyrone Woods, both former Navy SEALs, the State Department said.
CNN's Arwa Damon, Jomana Karadsheh, Elise Labott, Adam Levine and Brian Walker contributed to this report.