The US has started investigating the attack on its embassy in Benghazi to find out whether it was a planned attack or a spontaneous reaction against an alleged anti-Islam film.
The Benghazi attack carries the traits of "a planned, co-ordinated, well-executed military style event," said House Intelligence Committee Chairman Mike Rogers.
"I know that this is being investigated, and we're working with the Libyan government to investigate the incident. I would not want to speculate on that at this time," added White House press secretary Jay Carney.
The US is investigating the involvement of organised militant groups such as al-Qaida's North Africa unit, al-Qaida in the Islamic Maghreb, and Ansar al-Sharia, a militant group which supports the pan-Islamic terrorist organisation.
According to US officials, heavy firing started targeting the embassy building at around 22:00 local time on the first day of the attack, leaving the building in flames soon after. American security personnel were unable to reclaim the building until the morning, after several unsuccessful attempts.
US Ambassador to Libya Christopher Stevens and three other staff members including Sean Smith, a state department employee, were killed as protesters stormed the building.
A team of anti-terrorism marines is being sent to Libya to reinforce security for US staff in the wake of the attack.
President Barack Obama condemned the assault as "outrageous and shocking", adding: "Make no mistake. justice will be done."
"It is especially tragic that Chris Stevens died in Benghazi because it is a city that he helped to save," Obama continued, referring to Stevens' work in post-Gaddafi Libya.
Stevens, 52, who had already served two terms in Libya, was confirmed as America's ambassador to the country in early 2012.
"At the height of the Libyan revolution Chris led our diplomatic post in Benghazi. [Stevens] worked to support this young democracy," said Obama.
Meanwhile, protesters clashed with riot police as they returned to the US embassy in Cairo for a second evening of demonstrations against the anti-Islam film, entitled Innocence of Muslims.
Police fired tear gas shells to disperse the crowd and some of them were carrying petrol bombs, the BBC reported, quoting eye witnesses.