The United States on Wednesday vowed to track down those behind the killings of its ambassador to Libya and three other Americans amid a regional furor over a film mocking Islam's prophet.

"We will not waver in our commitment to see that justice is done for this terrible act," said President Barack Obama. "And make no mistake, justice will be done."

The slain ambassador, Chris Stevens, helped save Libya's eastern city of Benghazi during last year's revolution. He died there Tuesday night, along with another diplomat and two State Department security officers, when a mob stormed the U.S. Consulate and set it ablaze in what U.S. sources said Wednesday was a pre-planned attack.

It was one of several American diplomatic missions that faced protests after the online release of a film that ridiculed Muslims and depicted the Prophet Mohammed as a child molester, womanizer and ruthless killer. In the Egyptian capital Cairo, several men scaled the walls of the U.S. Embassy and tore down its American flag.

But the U.S. sources told CNN the attackers used the protest outside the consulate as a diversion, and Obama called the violence unwarranted.

"Since our founding, the United States has been a nation that respects all faiths. We reject all efforts to denigrate the religious beliefs of others," he said. "But there is absolutely no justification for this type of senseless violence -- none."

Libya's leaders apologized for the attack, with Prime Minister Abdurrahim el-Keib calling it a "cowardly, criminal act."

Sources tracking militant Islamist groups in eastern Libya say a pro-al Qaeda group responsible for a previous armed assault on the Benghazi consulate is the chief suspect.

Comment: al-Qaeda? More like Al-CIA-Duh.

A senior U.S. official told CNN that American surveillance drones are expected to join the hunt for jihadists who may be tied to the attack. The drones are expected to gather intelligence that will be turned over to Libyan officials for strikes, the official said.

Comment: More justification for killing innocent people. It is very likely we will be seeing an increase of drone kills in Libya for the next little while.

And a senior defense official told CNN the drones would be part of "a stepped-up, more focused search" for a particular insurgent cell that may have been behind the killings.

In June, a senior Libyan official told CNN that U.S. controllers were already flying the unmanned craft over suspected jihadist training camps in eastern Libya because of concerns about rising activity by al Qaeda and like-minded groups in the region.

The FBI also is investigating, the bureau said Wednesday.

Tuesday's attack took place on the 11th anniversary of the al Qaeda attacks on New York and Washington.

A senior U.S. official familiar with the details of the attack said a rocket-propelled grenade set the consulate ablaze, leaving the Americans facing a fire inside and attackers outside.

Stevens and the others who died were separated from the rest of the staff while trying to escape to the roof of the building. The official said there were several "valiant but unsuccessful" attempts to get back into the building and rescue them.

Stevens, Foreign Service information management officer Sean Smith and a U.S. regional security officer were in a safe room in the Benghazi post as it burned around them, according to CNN sources. The security official got out and went back for Stevens and Smith, but found Smith dead and recovered his body, the sources said.

Stevens managed to escape the burning house, but the sources did not know what his condition was when he got out. The sources said Smith died of smoke inhalation, but it was not clear how Stevens died.

American sources could not say whether the attackers instigated the protest or merely took advantage of it, and they say they don't believe Stevens was specifically targeted.

Secretary of State Hillary Clinton dispatched Stevens to Libya to be the American link to rebel forces battling to overthrow longtime Libyan strongman Moammar Gadhafi in 2011.

"He arrived on a cargo ship in the port of Benghazi and began building our relationships with Libya's revolutionaries," Clinton said Wednesday. "He risked his life to stop a tyrant, then gave his life trying to build a better Libya."

At a Washington news conference held by American religious leaders to condemn both the violence and the anti-Muslim film, Libya's ambassador to the United States called Stevens "the right man for the right position for the right time."

"He believed in the Libyan people," said Ambassador Ali Suleiman Aujali, who said Stevens was a personal friend. "He believed that America should support the Libyan people to get their country back. We need to practice democracy like other nations in the world."

Clinton said Smith was a 10-year veteran of the State Department, a husband and a father of two. The two other victims had not been named Wednesday afternoon, Clinton said the government was working to contact their families.

Clinton said their deaths are "not easy." But she added, "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. Everywhere Chris and his team went in Libya, in a country scarred by war and tyranny, they were hailed as friends and partners. And when the attack came yesterday, Libyans stood and fought to defend our post. Some were wounded. Libyans carried Chris' body to the hospital, and they helped rescue and lead other Americans to safety."

About 50 U.S. Marines from a rapid reaction force headed to Tripoli after the attack to beef up security in response to the attack, U.S. officials said Wednesday. The unit is specially trained to retake or guard diplomatic installations and other U.S. facilities in troubled regions.

The consulate was badly damaged and was being looted Wednesday, said a contractor working at the mission, who asked not to be named for security reasons. He said he saw the bodies of all four Americans on the street Wednesday morning.

Libyans also were killed, the contractor said, saying the victims were shot on the spot.

Libyan Deputy Interior Minister Wanis al-Sharif said consulate security staff opened fire after they heard gunfire outside the mission.

"This led to more anger, and this is when the consulate was stormed," he said. "Criminals managed to get in and they burned and ransacked the consulate."

He blamed the violence on people loyal to the late Gadhafi.

Stevens spoke Arabic and French and was among the first U.S. diplomats sent to Libya in 2007, when Washington and Tripoli resumed ties. He is the sixth U.S. ambassador to be killed in the line of service, while two others have died in plane crashes.

The last time an American ambassador was murdered was in 1979, when the envoy to Afghanistan, Adolph Dubs, was kidnapped and killed during an attempt to rescue him, according to State Department records.