For the first time in the much-politicized aftermath, the report also points at Ambassador Chris Stevens, who was killed in the attack. It says that the State Department ended a deal with the military to have a special operations team provide extra security in Libya, and that Stevens twice refused an offer to reinstate the team in the weeks before the Sept. 11, 2012, attack.
The military also takes criticism in the report for failing to respond more quickly on the night of the assault.
On the 11th anniversary of the 9/11 terror attacks in the U.S., armed militants stormed the diplomatic outpost in Benghazi, setting the building on fire. Stevens, information technology specialist Sean Smith, and CIA security contractors Tyrone Woods and Glen Doherty, both former Navy SEALs, were killed over the course of two battles that night.
Stevens died of smoke inhalation after he was taken to a "safe room" in the besieged compound. The Obama administration, reluctant to deal publicly with a terror attack weeks before the presidential election, first described the assault as a spontaneous mob protest of an anti-Islamic, American-made video. Such a protest did occur at the U.S. Embassy in Cairo earlier that day.
Officials corrected their description days after the attack, but by then it had become a hot political issue that has continued to dog the administration.
On that issue, the report dives into the contentious initial talking points issued by the intelligence community, which helped fuel Republican allegations of an Obama administration cover-up of militant links to the violence.
"Intelligence analysts inaccurately referred to the presence of a protest at the U.S. mission facility before the attack based on open source information and limited intelligence, but without sufficient intelligence or eyewitness statements to corroborate that assertion," the report said, adding that U.S. intelligence then took too long to correct the error.
The senators also take the administration to task for failing to bring the attackers to justice more than a year later. They say the U.S. has identified several individuals responsible but can't capture them because of limited intelligence capabilities in the region and limited cooperation by local governments.
Intelligence Committee Chairman Dianne Feinstein, a Democrat, said she hoped the report would put to rest conspiracy theories about the attacks.
Republican Vice Chairman Saxby Chambliss said the report showed that despite a deteriorating security situation in Benghazi, the U.S. government did not do enough to prevent the attacks or to protect the diplomatic facility. And Republican committee member Susan Collins of Maine called on the administration to punish those responsible.
"A broken system overseen by senior leadership contributed to the vulnerability of U.S. diplomats ... in one of the most dangerous cities in the world," she said in the report. "And yet the secretary of state has not held anyone responsible for the system's failings."
U.S. intelligence ultimately blamed the violence on militants who overran the temporary U.S. mission and, hours later, fired mortars at the nearby CIA annex where the Americans had taken shelter.
The report says the subsequent investigation showed individuals from many militant groups took part in the "opportunistic" attacks, including al-Qaida in the Islamic Maghreb, the Libyan militia group Ansar al-Sharia, and members of the Yemeni-based al-Qaida in the Arabian Peninsula.
The report does not name Hillary Rodham Clinton, who was secretary of state at the time and now is a potential 2016 presidential candidate.
The State Department said the report largely reaffirms the findings reached a year ago by the Benghazi Accountability Review Board, headed by a former ambassador and former chairman of the Joint Chiefs of Staff. Deputy spokeswoman Marie Harf noted that the Senate report recommended improvements to security that the State Department has already taken, including upgrading security cameras, improving fire equipment and increasing the presence of Marine security guards.
The Senate report notes that the State Department has also created a new assistant secretary position for high-threat posts, but it says the department still needs institutional change to help it react more quickly to security threats. It says State should not rely on local security alone in countries where the host government cannot provide adequate protection and should avoid using diplomatic facilities it knows are inadequately protected.
The report says that the department in 2012 had ignored its own "tripwires" set to determine when it had become too dangerous to operate in Benghazi, and continued to operate the facility there despite a growing number of U.S. intelligence reports showing the danger was rising.
"The security situation in Benghazi is 'trending negatively' and ... this daily pattern of violence would be the new normal for the foreseeable future," the head State Department officer in Benghazi was quoted as saying weeks before the attack. While the nearby CIA annex upgraded its security, the temporary mission did not, the report said.
It said Stevens acknowledged the need for more security yet also turned down available U.S. military resources. The report said the Defense Department had provided a Site Security Team in Tripoli, made up of 16 special operations personnel. But the State Department decided not to extend the team's mission in August 2012, one month before the attack. In the weeks that followed, Gen. Carter Ham, then the head of the military's Africa Command, twice asked Stevens to employ the team, and twice Stevens declined, the report said.
Stevens had tried to arrange a local Libyan security force to replace the Americans, but the report said the force was never formed because of bureaucratic delays.
Still, the report faults the U.S. military for failing to anticipate it might be called on. It also says there was confusion within the Pentagon as to the location of the CIA annex - and says the regional U.S. commander must know where such facilities are in the future.
White House spokesman Jay Carney said the committee report "largely reaffirms" earlier findings and that the security recommendations are consistent with steps the State Department has already taken.
"This reinforces what other investigations have found, which is that there was not enough security to protect the four Americans who lost their lives," Carney told reporters traveling with Obama Tuesday to North Carolina.
AP Writers Julie Pace and Deb Riechmann contributed to this report.