The review board cited a lack of resources as at least partly to blame for the deaths in Benghazi.
"The solution requires a more serious and sustained commitment from Congress to support State Department needs," it said.
The board found that Washington tended "to overemphasize the positive impact of physical security upgrades ... while generally failing to meet Benghazi's repeated requests" to beef up personnel.
The board completed its investigation this week and sent a copy Monday to Clinton, who said in letters to the heads of the Foreign Affairs and Foreign Relations Committees that she accepted all of its 29 recommendations. They include strengthening security, adding fire-safety precautions and improving intelligence collection in high-threat areas.
The report says "there was no protest prior to the attacks," which were described as "unanticipated in their scale and intensity."
It also cites the Bureau of Diplomatic Security staff as "inadequate" in Benghazi on the day of the attacks and in the months and weeks leading up to it, "despite repeated requests from Special Mission Benghazi and Embassy Tripoli for additional staffing."
The report says there had been a "lack of transparency, responsiveness, and leadership at the senior levels" in Washington, Tripoli and Benghazi.
"Security in Benghazi was not recognized and implemented as a 'shared responsibility' by the bureaus in Washington charged with supporting the post, resulting in stove-piped discussions and decisions on policy and security," it says. "That said, Embassy Tripoli did not demonstrate strong and sustained advocacy with Washington for increased security for Special Mission Benghazi."
The report says the short-term nature of the mission's staff, many of whom were inexperienced U.S. personnel, "resulted in diminished institutional knowledge, continuity and mission capacity."
The mission was also "severely under-resourced with regard to certain needed security equipment," it says.
It singles out for criticism dependence on "poorly skilled" members of the Libyan February 17 Martyrs' Brigade and unarmed local guards who were supposed to provide security. It noted that, at the time of Stevens' visit, militia members "had stopped accompanying Special Mission vehicle movements in protest over salary and working hours."
Though it said there had been no specific, credible threats on the day of the attack, the significance of the anniversary of the 9/11 terrorist attacks in 2001 had led Stevens to hold meetings on the compound on Sept. 11 of this year.
But security systems and the Libyan response "fell short" when the compound was penetrated "by dozens of armed attackers."
The report presents a detailed description of what happened that night. It says Libyan mission guards were not present, local militia fled their posts and "there simply was not enough time for armed U.S. military assets to have made a difference."
The board said it could not determine how a gate at the compound was breached, "but the speed with which attackers entered raised the possibility" that the guards had left it open.
The politics surrounding the events that led to the report have claimed one political casualty, with Susan Rice, U.S. ambassador to the United Nations, last week pulling her name from consideration to succeed Clinton. Some Republican senators had said they would put a hold on her nomination if President Barack Obama had submitted it, based on comments, later determined to have been inaccurate, that Rice made in the days after the attack.