Pentagon spokesman George Little said, "Without commenting on specific ship movements, the United States military regularly takes precautionary steps when potential contingencies might arise in a given situation. That's not only logical in certain circumstances, it's the prudent thing to do."
At the same time, some 50 U.S. Marines headed to Libya to reinforce security at U.S. diplomatic facilities, initially at the American embassy in Tripoli, not Benghazi.
The officials spoke on condition of anonymity because they were not authorized to publicly discuss troop movements.
White House press secretary Jay Carney said it was too early to judge whether the Benghazi attack was planned.
"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," he said. Several Libyan security guards also were killed.
Rogers, R-Mich., said U.S. intelligence had not yet determined who was responsible, but added, "Our list is narrowing."
"When you see [such an attack], it wasn't some folks who had some guns in their garage and said let's shoot up the consulate," Rogers said in an interview Wednesday.
The FBI was sending evidence teams to Libya, said a law enforcement official.
Analysts are working on several different scenarios based on intelligence that could lead to a motive for the attack. Some concern the possibility of targeting high-ranking officials, according to a law enforcement official briefed on the investigation. But none of the intelligence has suggested terrorists would specifically target Stevens, said the official who also spoke on condition of anonymity because he was not authorized to speak publicly about the investigation.
The attack in Libya, which came hours after a mob stormed the U.S. Embassy in Cairo and tore down the U.S. flag, was presumed to have been triggered by a movie, whose trailer has gone viral on YouTube, depicting the Islamic prophet Muhammad in disrespectful ways. In an extraordinary move, Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, called anti-Islamic preacher Terry Jones and asked him to stop promoting the film. A spokeswoman said the church would not show the film Wednesday evening.
"Make no mistake. Justice will be done," a somber Obama pledged at the White House, Secretary of State Hillary Rodham Clinton at his side.
He ordered increased security at U.S. diplomatic missions overseas, particularly in Libya, and said he condemned "in the strongest possible terms the outrageous and shocking" attack. Clinton said she was particularly appalled that the attack took place in Benghazi, which the U.S. had helped liberate from dictator Moammar Gadhafi during the Arab Spring revolution in Libya last year.
Three Americans were wounded, U.S. officials said.
The aftermath of the two attacks also stirred the U.S. presidential campaign, where until Wednesday, foreign policy had taken a back seat to the struggling economy.
The Republican nominee, Mitt Romney, criticized the administration for statements issued before and after the Cairo attacks that expressed sympathy for those insulted by the video. But he in turn was criticized for turning a tragedy too quickly into political fodder, and getting key details wrong. And his account didn't mesh completely with events in Cairo.
The embassy statement that he referred to as akin to an apology was issued at midday on Tuesday in Cairo at a time the embassy staff was aware of still-peaceful demonstrations in the area nearby. It was four or five hours later when the mob breached the compound's walls and tried to burn a U.S. flag.
Obama told CBS' "60 Minutes" that Romney "seems to have a tendency to shoot first and aim later."
Obama and Clinton made a rare joint visit to the State Department, where grieving colleagues of Stevens and the other three Americans killed in Benghazi gathered in a courtyard. The president also ordered U.S. flags to be flown at half-staff at government and military buildings and vessels around the world until sunset on Sept. 16. Flags had already been lowered in many places to commemorate the victims of the 9/11 attacks.
Clinton denounced those who might kill over an insulting movie.
"There is no justification for this," Clinton said. "None. 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."
Underscoring the administration's frustration, Clinton wondered aloud about the attack in Benghazi, which Gadhafi had once threatened to destroy.
"This is not easy," she said. "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, just how confounding, the world can be."
"But we must be clear-eyed in our grief," she said, saying the attack was carried out by a "small and savage group" not representative of the Libyan people. She noted that Libyan security guards had tried to fight off the attackers, had carried Stevens' body to the hospital and led other consulate employees to safety.
Stevens, a 52-year-old career diplomat, was killed after he became separated from other American officials during the consulate attack. It's unclear when he died: He was taken by Libyans to a hospital, and his remains were delivered hours later to U.S. officials at the Benghazi airport.
Stevens is the first U.S. ambassador to be killed in an attack since 1979, when Ambassador Adolph Dubs was killed in Afghanistan.
Three other Americans were also killed and the State Department identified one of them as Sean Smith, an Air Force veteran who had worked as an information management officer for 10 years in posts such as Brussels, Baghdad and Pretoria. Smith was also well-known in the video game community.
The identities of the others were being withheld pending notification of relatives.
"The mission that drew Chris and Sean and their colleagues to Libya is both noble and necessary, and we and the people of Libya honor their memory by carrying it forward," Clinton said.
Stevens spoke Arabic and French and had already served two tours in Libya, including running the office in Benghazi during the revolt against Gadhafi. He was confirmed as ambassador to Libya by the Senate earlier this year. Huntsman salutes ambassador killed in Libya
Former Utah Gov. Jon Huntsman, who has twice served as a U.S. ambassador, expressed condolences on the killing of U.S. Ambassador to Libya Christopher Stevens. Huntsman, who did not know Stevens personally, issued a statement lauding Stevens for "representing the highest ideals of American diplomacy."
"This is a time when we all should reflect on those who continue to give, even the last measure, of service and sacrifice, to promoting and defending America's interests abroad," Huntsman added. "This is above all a reminder that politics should end at the waters' edge."
Huntsman's last sentence appears to be directed at Republican presidential candidate Mitt Romney, the only top Republican who has condemned President Barack Obama's administration for what Romney said was apologizing for America in the government's initial response.
Critics, including some former Republican State Department officials, have slammed Romney for politicizing the tragedy. Obama, Clinton statements on Libya attack
Statements by President Barack Obama and Secretary of State Hillary Rodham Clinton on Tuesday's attack on the U.S. consulate in Benghazi, Libya, as released Wednesday by the White House:
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. They exemplified America's commitment to freedom, justice, and partnership with nations and people around the globe, and stand in stark contrast to those who callously took their lives.
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.
On a personal note, Chris was a courageous and exemplary representative of the United States. Throughout the Libyan revolution, he selflessly served our country and the Libyan people at our mission in Benghazi. As Ambassador in Tripoli, he has supported Libya's transition to democracy. His legacy will endure wherever human beings reach for liberty and justice. I am profoundly grateful for his service to my Administration, and deeply saddened by this loss.
The brave Americans we lost represent the extraordinary service and sacrifices that our civilians make every day around the globe. As we stand united with their families, let us now redouble our own efforts to carry their work forward.
It is with profound sadness that I share the news of the death of four American personnel in Benghazi, Libya, yesterday. Among them were United States Ambassador to Libya Chris Stevens and Foreign Service Information Management Officer Sean Smith. We are still making next of kin notifications for the other two individuals. Our hearts go out to all their families and colleagues.
A 21-year veteran of the Foreign Service, Ambassador Stevens died last night from injuries he sustained in the attack on our office in Benghazi.
I had the privilege of swearing in Chris for his post in Libya only a few months ago. He spoke eloquently about his passion for service, for diplomacy and for the Libyan people. This assignment was only the latest in his more than two decades of dedication to advancing closer ties with the people of the Middle East and North Africa, which began as a Peace Corps Volunteer in Morocco. As the conflict in Libya unfolded, Chris was one of the first Americans on the ground in Benghazi. He risked his own life to lend the Libyan people a helping hand to build the foundation for a new, free nation. He spent every day since helping to finish the work that he started. Chris was committed to advancing America's values and interests, even when that meant putting himself in danger.
Sean Smith was a husband and a father of two, who joined the Department ten years ago. Like Chris, Sean was one of our best. Prior to arriving in Benghazi, he served in Baghdad, Pretoria, Montreal and most recently The Hague.
All the Americans we lost in yesterday's attacks made the ultimate sacrifice. We condemn this vicious and violent attack that took their lives, which they had committed to helping the Libyan people reach for a better future.
America's diplomats and development experts stand on the front lines every day for our country. We are honored by the service of each and every one of them.