JUDY WOODRUFF: Questions swirled today about the death of the U.S. ambassador to Libya and its implications.
He died last night in the eastern city of Benghazi after thousands of people surrounded and then attacked the American Consulate there.
Margaret Warner begins our coverage.
MARGARET WARNER: U.S. Ambassador Chris Stevens was the first American envoy to die in the line of duty in more than 30 years. He and three other State Department officers were killed in the assault Tuesday night in Benghazi.
Stevens had been trying to evacuate staffers from the U.S. Consulate when gunmen with automatic weapons and rocket-propelled grenades stormed the lightly guarded compound and set it on fire.
The identity of the attackers and their motivations remained murky. But, in Washington, White House officials said militants tied to al-Qaida may have used protests against an anti-Islam film as a diversion.
This morning, President Obama, with Secretary of State Clinton at his side, praised the slain ambassador.
PRESIDENT BARACK OBAMA: It's especially tragic that Chris Stevens died in Benghazi, because it is a city that he helped to save.
At the height of the Libyan revolution, Chris led our diplomatic post in Benghazi. With characteristic skill, courage and resolve, he built partnerships with Libyan revolutionaries and helped them as they planned to build a new Libya.
MARGARET WARNER: Flags over the White House, the Capitol and the State Department were lowered to half-staff, and tributes to Stevens poured in.
A Middle East veteran fluent in Arabic and French, Stevens had been on the job since May, introducing himself to the Libyan people via YouTube.
CHRISTOPHER STEVENS, U.S. Ambassador to Libya: I look forward to exploring those possibilities with you as we work together to build a free, democratic, prosperous Libya. See you soon.
MARGARET WARNER: Killed alongside Stevens was Sean Smith, a State Department officer, and two Americans as yet unidentified.
In her own statement today, Secretary Clinton condemned the attacks in the wake of U.S. support for the Libyan revolution.
SECRETARY OF STATE HILLARY RODHAM CLINTON: Many Americans are asking -- indeed, I asked myself, how could this happen? How could this happen in a country we helped liberate, in a city we helped save from destruction?
This question reflects just how complicated and, at times, how confounding the world can be. But we must be clear-eyed even in our grief. This was an attack by a small and savage group, not the people or government of Libya.
MARGARET WARNER: The president said the United States would work with the Libyan government to track down the perpetrators.
BARACK OBAMA: Today, we mourn for more Americans who represent the very best of the United States of America. We will not waver in our commitment to see that justice is done for this terrible act. And make no mistake, justice will be done.
MARGARET WARNER: For now, the Pentagon ordered special units of Marines to Libya called FAST teams, like this detachment shown training, to reinforce security at diplomatic sites in Libya.
And from Tripoli, the president of Libya's National Assembly echoed the words of his American counterparts.
MOHAMMED MAGARIEF, Libyan National Assembly (through translator): We apologize to the United States of America and to the American people and to the whole world for what happened, and at the same time we expect the rest of the world to help us face these cowardly criminal acts.
We refuse to use our country's land as a scene of cowardly reprisals.
MARGARET WARNER: Those reprisals came apparently in response to Internet clips of a film titled "The Innocence of Muslims" that crudely defamed the Prophet Mohammed. A California man calling himself Sam Bacile has said he produced the movie.
It was also promoted recently by Florida preacher Terry Jones, whose threats to burn the Koran led to widespread chaos and deaths in Afghanistan two years ago. But recent coverage of the film in Egyptian media helped propel the protests, which started in Cairo several days ago.
Conservative Islamists, some who have long camped outside the U.S. Embassy, scaled the compound walls yesterday and tore down the American flag. They replaced it with a black banner proclaiming "There is no God but God, and Mohammed is his prophet."
Egyptian authorities announced today they have arrested four people in connection with that rioting. The new Egyptian president, Mohammed Morsi, has asked American authorities to take action against the filmmaker, but he has not so far condemned the attack on the U.S. Embassy.
Afghan President Hamid Karzai also criticized the film, without mentioning the violence.
And the Afghan Taliban urged Muslims to attack U.S. troops in revenge. Hours before the attack, the U.S. Embassy in Cairo decried what it called "efforts by misguided individuals to hurt the religious feelings of Muslims."
That drew criticism from Republican presidential nominee Mitt Romney. In a statement last night, he charged: "The Obama administration's first response was not to condemn attacks on our diplomatic missions, but to sympathize with those who waged the attacks."
The Obama campaign rebuked Romney, but the candidate reiterated his statement this morning in Jacksonville, Fla.
MITT ROMNEY (R): I think it's a -- a -- a terrible course to -- for America to -- to stand in apology for our values, that, instead, when our grounds are being attacked and being breached, that the first response of the United States must be outrage at the breach of the sovereignty of our nation.
And apology for America's values is never the right course.
MARGARET WARNER: The president visited the State Department and met with diplomats and staff.
Later, he told CBS News: "Gov. Romney seems to have a tendency to shoot first and aim later. And as president, one of the things I have learned is you can't do that."