The US ambassador to Libya, Christopher Stevens, was among four Americans killed in an attack on the American consulate in Benghazi on Tuesday.
In a statement confirming the deaths, President Barack Obama said:
"I strongly condemn the outrageous attack on our diplomatic facility in Benghazi, which took the lives of four Americans, including Ambassador Chris Stevens. Right now, the American people have the families of those we lost in our thoughts and prayers. I have directed my Administration to provide all necessary resources to support the security of our personnel in Libya, and to increase security at our diplomatic posts around the globe.
"While the United States rejects efforts to denigrate the religious beliefs of others, we must all unequivocally oppose the kind of senseless violence that took the lives of these public servants."
Speaking from the White House hours later, Obama once more emphasized the need for unity: "The world must stand together to unequivocally reject these brutal acts."
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Secretary of State Hillary Clinton said the attack should "shock the conscience of people of all faiths around the world." In a live webcast from the State Department, Clinton said that there was "no justification for this. None."
On Wednesday, Obama adminstration officials said that they suspected the attack had been planned in advance, using protest as a guise to make the violence appear spontaneous, The New York Times reported.
Reuters reported that Libya's Deputy Interior Minister, Wanis Al-Sharif, said two of the four embassy personnel killed on Tuesday were shot in a separate attack on a consular safe house, where staff had been moved after the attack on the consulate.
All US personnel were evacuated from Benghazi Wednesday, while staff at the US Embassy in Libya's capital, Tripoli, were reduced to "emergency" levels, Reuters reported, quoting a senior US official.
Clinton blamed the violence on a "small and savage group" of extremists rather than the Libyan government or people, several of whom, she said, had attempted to protect the US consulate and were injured as a result.
The secretary of state appeared determined to stress unity between the US and Libya, pledging that "the friendship between our countries born out of shared struggle will not be another casualty of this attack. We will not turn our back on that, nor will we rest until those responsible for these attacks are found and brought to justice."
Obama said that the US government would work with Libyan authorities to pursue the perpetrators.
Libyan Deputy Prime Minister Mustafa Abushagour deplored the violence.
"I condemn the cowardly act of attacking the US consulate and the killing of Mr. Stevens and the other diplomats," the Wall Street Journal cited Abushagour as saying.
"This is an attack on America, Libya and free people everywhere."
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A Libyan official told Reuters that Stevens and the others were being rushed from a consular building as it was stormed by militants angered over a US-made film that they say insults the Prophet Mohammed.
The film, produced by Sam Bacile, an Israeli film-maker living in California, has sparked angry protests in Egypt and Yemen after excerpts dubbed into Arabic were posted on YouTube.
Bacile told the Wall Street Journal that he made the film with $5 million he received from 100 Jewish donors. He added: "Islam is a cancer. The movie is a political movie. It's not a religious movie."
The film was reportedly being promoted in the US by an anti-Muslim, Egyptian Christian campaigner.
Stevens is usually based in the capital, Tripoli, but was apparently visiting Benghazi ahead of the planned opening of an American cultural center there.
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Here is a graphic on how US politicans have discussed the attacks, and each other's reactions.