President Barack Obama on Wednesday denounced the "outrageous and shocking attack" that claimed the lives of the U.S. ambassador to Libya, Chris Stevens, and three other Americans and vowed to work with that country's fledgling government to bring the perpetrators to justice.

"The United States condemns in the strongest terms this outrageous and shocking attack," Obama said with Secretary of State Hillary Clinton at his side in hastily arranged public remarks in the Rose Garden of the White House.

"Today, we mourn four 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."

Stevens lost his life in an assault on the American diplomatic mission in Benghazi, Libya, amid anger among Islamist extremists at a low-quality film mocking the Muslim Prophet Muhammad. Outrage at the movie also led an angry phalanx of Egyptian protesters to storm the U.S. Embassy there and tear down its American flag.

But U.S. officials told the New York Times and CNN that this was no protest turned violent but a planned strike by an organized group that exploited, or even organized, the demonstrations. (The timing—September 11—lent weight to that scenario.)

Still, noting anger in the Middle East at the film, Obama declared, "We reject all efforts to denigrate the religious beliefs of others. But there is no justification for this senseless violence—none.

"The world must stand together to unequivocally reject these brutal acts," Obama said. "Already, many Libyans have joined us in doing so, and this attack will not break the bonds between the United States and Libya."

Both Obama and Clinton released written statements earlier condemning the attack on the American mission in Benghazi.

The president's public appearance came after Mitt Romney spoke out against both attacks and leveled the inflammatory charge that the Obama administration's "first response was not to condemn attacks on our diplomatic missions, but to sympathize with those who waged the attacks."

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Republicans said their standard-bearer was referring to a statement from the U.S. Embassy in Egypt condemning the anti-Islam film that demonstrators there denounced as they attacked the facility, bringing down the American flag. That statement—which was released before Stevens' death—did not apologize for the film or express sympathy for the protests, though it did denounce "the continuing efforts by misguided individuals to hurt the religious feelings of Muslims."

Speaking at an impromptu press conference in Jacksonville, Fla., Romney condemned Tuesday's attacks as "disgusting" and "outrageous" and accused the Obama administration of weakness.

"It's a terrible course for America to stand in apology for our values," Romney told reporters. "It's never too early for the U.S. government to condemn attacks on Americans and defend our values. ... When our grounds are being attacked, being breached, the first response of the United States must be outrage."

Obama did not directly address Romney's criticisms. Neither did Clinton, who described the deadly clashes in more detail in separate remarks at the State Department earlier in the day.

"Heavily armed militants assaulted the compound and set fire to our buildings. American and Libyan security personnel battled the attackers together," she said.

"We are working to determine the precise motivations and methods of those who carried out this assault," Clinton said. But "violence like this is no way to honor religion or faith. And as long as there are those who would take innocent life in the name of God, the world will never know a true and lasting peace."

And Clinton paid a solemn, personal tribute to Stevens.

"In the early days of the Libyan revolution, I asked Chris to be our envoy to the rebel opposition," she said. "He arrived on a cargo ship in the port of Benghazi and began building our relationships with Libya's revolutionaries. He risked his life to stop a tyrant, then gave his life trying to help build a better Libya. The world needs more Chris Stevenses.

"Today, 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," Clinton said.

"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," she said, vowing that "the friendship between our countries, borne out of shared struggle, will not be another casualty of this attack.

"A free and stable Libya is still in America's interest and security, and we will not turn our back on that, nor will we rest until those responsible for these attacks are found and brought to justice," she said.

Obama later traveled to the State Department to offer his condolences to rattled and distraught foreign service officers.

Stevens, 52, was the first U.S. ambassador killed in the line of duty since 1979, when the envoy to Afghanistan, Adolph Dubs, was shot in a kidnapping attempt.

Obama was briefed Tuesday about the violence in Egypt and Libya by national security adviser Tom Donilon as he met with Defense Secretary Leon Panetta and Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff.

The president got several updates throughout the evening and again this morning, according to a White House official.

"The president was notified last night that Ambassador Stevens was unaccounted for and then notified again this morning about his tragic death," the official said.