The top White House anti-terror official John Brennan pressed Libyan leaders on Wednesday to hunt down those guilty of killing Ambassador Chris Stevens and three other Americans as controversy mounted in Washington.
Just four weeks ahead of the US presidential election, Barack Obama's administration faced mounting criticism from Republicans over the breach of security at the US consulate in Libya's second city Benghazi on September 11 that led to the killings.
Security was tight in the Libyan capital for Brennan's talks with national assembly chief Mohammed Megaryef, Libya's de facto head of state, and other senior officials.
Brennan set out to Megaryef "specific additional steps Libya can take to better assist the US in ensuring that the perpetrators are brought to justice," the White House said.
"Mr Brennan encouraged Libyan officials to move quickly on refining their policies and advancing government capabilities in the security and justice sectors," National Security Council spokesman Tommy Vietor said.
Brennan also urged Libya to "take full and timely advantage of specific offers of assistance from the United States and other international partners."
The envoy's visit came with political temperatures rising in Washington as senior State Department officials gave evidence to a congressional hearing into the deadly consulate attack and Obama's challenger Mitt Romney went on the offensive.
Claims by Republicans that the White House covered up a terrorist element in the assault and that there was lax security in Benghazi have been drawn into the campaign.
The day after the assault, the Libyan authorities promised their full cooperation with a US inquiry into the attack but security conditions in Benghazi have meant that it was only on October 4 that an FBI team probing the killings was finally able to visit the scene.
Benghazi was the cradle of the 2011 revolt that toppled and killed long-time dictator Moamer Kadhafi but has since become the focus of mounting violence, some of it involving Islamist or even jihadist militias.
In a dramatic new account, two State Department officials Tuesday described a relentless attack in which dozens of armed men invaded the US consulate in Benghazi, setting it on fire and hunting through the building for staff.
There had been no warning that an attack was planned, and in the hours before the streets outside the compound had been calm, they said on a conference call with reporters, asking to remain anonymous.
The new account contradicts initial reports by State Department officials which said it was a "spontaneous" attack sparked by a protest against an anti-Islam film.
On Wednesday, the former commander of a special security team in Tripoli, told a US congressional hearing that diplomatic security in Benghazi was weak and deteriorating and that Al-Qaeda's presence in Libya was growing.
Al-Qaeda's "presence grows every day" in Libya, Lieutenant Colonel Andrew Wood told the hearing of the House Oversight and Government Reform committee into the US consulate attack.
"They are certainly more established than we are," said Wood, who headed up a 16-strong site security team in Tripoli from mid-February to mid-August.
A US official has said that they are investigating whether Al-Qaeda in the Islamic Maghreb may have been behind the Benghazi attack.
In Benghazi, "the situation remained uncertain and reports from some Libyans indicated it was getting worse. Diplomatic security remained weak," he said.
He said targeted attacks on Westerners were on the rise and fighting between militias common in the eastern city.
In June there had been a direct threat made against ambassador Stevens on Facebook, mentioning that he liked to jog regularly, he said, adding that in April there was only one US diplomatic security agent based in Benghazi.
Wood said he visited Benghazi twice and was there in June when the British ambassador's convoy was attacked and had helped provide medical and security help afterwards.