A Senate Intelligence Committee inquiry held hearings and interviewed dozens of witnesses, against a back-drop of partisan point-scoring from both sides of Washington's political divide.
Some Republican lawmakers accuse President Barack Obama of concealing evidence that Al Qaeda-linked jihadi groups were behind the outrage and of failing to properly protect the outpost.
Obama's administration initially suggested the attacks were a spontaneous protest by Benghazi residents angered by a privately-made American anti-Islamic film posted online.
Wednesday's bipartisan report emphasized the security shortfalls that allowed protesters and armed militants to storm Stephen's Benghazi compound and to torch the US residence.
The report said the State Department had failed to heed warnings to reinforce protection at the sites despite the rapidly deteriorating security environment in Libya.
And it blamed intelligence agencies for not notifying US military officials in the US Africa command that a CIA annex even existed near the Benghazi diplomatic mission.
"The committee found the attacks were preventable, based on extensive intelligence reporting on the terrorist activity in Libya -- to include prior threats and attacks against Western targets -- and given the known security shortfalls at the US mission," the panel said in a statement.
"The State Department should have increased its security posture more significantly in Benghazi based on the deteriorating security situation on the ground and IC (intelligence community) threat reporting."
The Obama administration appeared to accept the Senate report, with White House spokesman Jay Carney saying it "largely reaffirms" the findings from the independent Benghazi Accountability Review Board of 13 months ago.
"The administration is focused on two pieces -- bringing to justice those responsible for the deaths of four Americans and making sure we take steps necessary to improve security at vulnerable facilities," Carney told reporters aboard Air Force One.
The 85-page report also attempts to clarify confusion surrounding the Obama administration's initial statements about the attacks, when it "inaccurately" referred to a protest at the US mission prior to the assault "without sufficient intelligence or eyewitness statements to corroborate that assertion."
Intelligence leaders then took "too long to correct these erroneous reports," adding to public confusion about what happened.
The report warns that the FBI probe has been stymied in Benghazi, where "as many as 15 individuals supporting the investigation or otherwise helpful to the United States have been killed."
Echoing the White House, the State Department, said the report reaffirms the earlier review board's assessments.
But deputy State Department spokeswoman Marie Harf also said that -- despite knowing there were "bad guys operating in Benghazi" -- it would be difficult to look back and determine exactly what could have been prevented.
"We had no specific information indicating a threat -- an attack was coming," she told reporters.
"We can't go back and look at the hypotheticals about what could have been prevented and what couldn't have. We have said that security needs to be improved. We've been crystal clear about that."
The report will fuel controversy over the responsibility of the State Department, which at the time of the attacks then secretary of state Hillary Clinton, and more broadly the Obama administration, which has identified several individuals responsible for the attacks but has failed to bring them to justice.
Several conservative lawmakers voiced their concern about the report's findings, including Senate Republican Marco Rubio, who said it provided only a partial picture of what happened.
Rubio, like Clinton a potential 2016 presidential candidate, said the Senate Foreign Relations Committee, of which he is a member, "should reexamine... Clinton's failure to provide adequate security for our deployed personnel in Benghazi, as well as what actions she and others, including the president, took in the hours and days that followed the attack."