Chris Stevens - the US Ambassador to Libya - has been killed in a terror attack.
According to a Libyan official he was among four embassy staff who died yesterday by protesters were firing guns and rocket propelled grenades during an attack in Benghazi.
"The Libyan ambassador and three staff members were killed when militia attacked the embassy," the official said.
Armed gunmen attacked the compound on yesterday evening, clashing with Libyan security forces before the latter withdrew as they came under heavy fire.
It is reported he died from suffocation.
Reporters on the scene said they could see looters raiding the building, walking off with desks, chairs and washing machines.
The film-maker whose movie attacking Islam's prophet Mohammed sparked the assaults on US missions in Egypt and Libya, said today he had gone into hiding.
But writer and director Sam Bacile, who spoke on the phone from an unidentified location, remained defiant, saying Islam was a cancer and he wanted his film to make a political statement.
The 56-year-old identifies himself as an Israeli Jew and says he believes his video will help his native land by exposing Islam's flaws to the world. Excerpts dubbed into Arabic were posted on YouTube.
Among other claims that have caused outrage, the film claims Mohammed was a philanderer who approved of child sex abuse.
Mr Bacile said he was sorry for the person who died, but blamed lax embassy security.
In Cairo, Egypt, protesters scaled the US embassy walls and replaced an American flag with an Islamic banner.
It was the first such assaults on US diplomatic buildings in either country, at a time when both Libya and Egypt are struggling to overcome the turmoil following the ousting of their long-time leaders Muammar Gaddafi and Hosni Mubarak in uprisings last year.
In Benghazi, a large mob stormed the US consulate, with gunmen firing their weapons, said Wanis al-Sharef, an Interior Ministry official. A witness said attackers fired automatic weapons and rocket-propelled grenades at the consulate as they clashed with Libyans hired to guard the building.
Outnumbered by the crowd, Libyan security forces did little to stop them, Mr al-Sharef said.
The crowd overwhelmed the consulate and set fire to it, burning most of it and looting the contents, witnesses said.
Hours before the Benghazi attack, hundreds of mainly ultra-conservative Islamist protesters in Egypt marched to the US embassy in Cairo, gathering outside its walls and chanting against the movie and the US. Most of the embassy staff had left the compound earlier because of warnings of the upcoming demonstration.
"Say it, don't fear: Their ambassador must leave," the crowd chanted.
Dozens of protesters then scaled the embassy walls, and several went into the courtyard and took down the American flag from a pole. They brought it back to the crowd outside, which tried to burn it, but failing that, tore it apart.
The protesters on the wall then raised on the flagpole a black flag with a Muslim declaration of faith, "There is no god but God and Mohammed is his prophet". The flag, similar to the banner used by al Qaida, is commonly used by ultra-conservatives around the region.
The crowd grew throughout yesterday evening with thousands standing outside the embassy. Dozens of riot police lined up along the embassy walls but did not stop protesters as they continued to climb and stand on the wall - though it appeared no more went into the compound.
A group of women in black veils and robes chanted: "Worshippers of the cross, leave the prophet Mohammed alone."
By midnight, the crowd had dwindled. The US embassy said on its Twitter account that there would be no visa services today because of the protests.
A senior Egyptian security official at the embassy area said authorities allowed the protest because it was "peaceful".
When they started climbing the walls, he said he called for more troops, denying that the protesters stormed the embassy.
The Cairo embassy is in a diplomatic area in Garden City, where the British and Italian embassies are located, only a few blocks away from Tahrir Square, the centre of last year's uprising that led to the ousting of Hosni Mubarak.
The Egyptian Foreign Ministry promised to provide the necessary security for diplomatic missions and embassies and warned that "such incidents will negatively impact the image of stability in Egypt, which will have consequences on the life of its citizens".
"This is a very simple reaction to harming our prophet," said protester Abdel-Hamid Ibrahim.
Speaking before reports of the murdered American emerged, US State Department spokeswoman Victoria Nuland condemned the attack on the consulate in Libya "in the strongest terms".
Muslims find it offensive to depict Mohammed in any fashion, much less in an insulting way. The 2005 publication of 12 caricatures of the prophet in a Danish newspaper triggered riots in many Muslim countries.
A 14-minute trailer of the film that sparked the protests, posted on the website YouTube in an original English version and another dubbed into Egyptian Arabic, depicts Mohammed as a fraud, a womaniser and a madman in an overtly ridiculing way, showing him having sex and calling for massacres.
A YouTube spokesman said the website's policy was to remove videos that include a threat of violence, but not those expressing only opinions.