Eric Nordstrom, the regional security officer in Libya until July, had conveyed concerns about the Libyan government's ability overall to protect American diplomatic facilities.

Moreover, he sent two cables to State Department headquarters in March and again in July requesting additional security agents for the Benghazi post, but did not receive any response, according to a summary of his interview with House Oversight and Government Reform Committee Democrats that was obtained by CNN.

Nordstrom told the committee that Deputy Assistant Secretary Charlene Lamb wanted to keep the number of U.S. security personnel in Benghazi "artificially low." Nordstrom said she generally believed that extra security was unnecessary because there was a residential safe haven to fall back on in an emergency, according to the summary.

She thought the "best course of action was to assign three agents" to the Benghazi post, the summary quoted Nordstrom as saying.

Nordstrom said the facility usually had three or four agents. But he also told the committee that Undersecretary of State for Management Patrick Kennedy issued a memo in December requiring five agents be assigned to Benghazi.

Senior State Department officials have said security personnel in Benghazi were outmanned and outgunned during the attack and that no reasonable security presence could have successfully fended it off.

"The lethality and the number of armed people is unprecedented. There had been no attacks like that anywhere in Libya -- Tripoli, Benghazi or anywhere -- in the time that we had been there. And so it is unprecedented, in fact, it would be very, very hard to find precedent for an attack like (this) in recent diplomatic history," a senior agency official said.

Who was behind the attack?

A pro-al Qaeda group responsible for a previous armed assault on the U.S. Consulate in Benghazi is the chief suspect in the attack, sources tracking militant Islamist groups in eastern Libya say.

They also note that the attack immediately followed a call from al Qaeda leader Ayman al-Zawahiri for revenge for the death in June of Abu Yahya al-Libi, a senior Libyan member of the terror group.

The group suspected to be behind the assault, the Imprisoned Omar Abdul Rahman Brigades, first surfaced in May when it claimed responsibility for an attack on the International Red Cross office in Benghazi. The next month the group claimed responsibility for detonating an explosive device outside the U.S. Consulate and later released a video of that attack.

Noman Benotman, once a leading member of the Libyan Islamic Fighting Group and now based at the Quilliam Foundation in London, said, "An attack like this would likely have required preparation. This would not seem to be merely a protest which escalated."

Benotman, who had earlier warned of the likelihood of renewed attacks against U.S. interests in Libya, said on Tuesday that the Omar Abdul Rahman Brigades is a prime suspect in the Benghazi attack. He said he believes it is likely the deadly attack was also linked to a video statement released by al-Zawahiri on the 11th anniversary of the 9/11 attacks. In the video, al-Zawahiri confirmed the death of al-Libi, a prominent member of the al Qaeda-linked group, adding: "His blood is calling, urging and inciting you to fight and kill the crusaders."

No names have been released for those responsible for carrying out the attack.

However, a series of arrests was made in the wake of the attack, and after President Obama said he would not rest until those responsible were brought to justice.

Mohamed al-Magariaf, the head of Libya's General National Congress, said around 50 people were arrested, though another senior government official said the number was not that high.

The official said as many as 50 people have been brought in for questioning, but not all of them were arrested. They were people who were at a protest outside the consulate, but there was no indication yet that they took part in the violence, he said.

Those arrests were confirmed by Mustafa Abushagur, the prime minister of Libya, who added that other people may have been involved and the investigation is ongoing.

Two Tunisians were also detained for questioning in Turkey, and U.S. Secretary of Defense Leona Panetta said there was a connection between the arrests and the attack.

"We know there is some connection, but, frankly, we really don't have all the specifics," Panetta said.

The Tunisians, who had been on a watch list provided by the United States to Turkish authorities, were being questioned at the request of the United States, a U.S. official with knowledge of the investigation said.