WASHINGTON (CNN) -- The United States is struggling to determine whether a militant group planned the attack that killed its ambassador to Libya and three other Americans, even as warships head toward the north African country as part of a mission to hunt down and punish the killers.
Conflicting theories flew in the hours after U.S. Ambassador Chris Stevens, another diplomat and two State Department security officers were killed in Benghazi, eastern Libya, late Tuesday.
They died amid a protest outside the U.S. Consulate over a film that ridiculed Muslims and depicted the Prophet Mohammed as a child molester, womanizer and ruthless killer.
The demonstration was one of several protests across the region that day.
But U.S. sources said Wednesday the four-hour assault in Benghazi had been planned by militants, with the attackers using the protest as a diversion.
A U.S. intelligence official told CNN the picture is becoming clearer within the intelligence community as to what group or groups were responsible for the attack. Given what officials know about al Qaeda in Libya, U.S. intelligence believes it is very unlikely that core al Qaeda was behind the attack, the official said.
Officials are not yet ready to identify a group.
State Department Under Secretary Patrick Kennedy offered his opinion Wednesday that the attack was planned because it was so extensive and because of the "proliferation" of small and medium weapons at the scene. He was briefing congressional staffers when he made the suggestion.
And a London think tank with strong ties to Libya echoed the theory.
The assault "came to avenge the death of Abu Yaya al-Libi, al Qaeda's second in command killed a few months ago," the think tank Quilliam said Wednesday.
It was "the work of roughly 20 militants, prepared for a military assault," the think tank said, noting that rocket-propelled grenade launchers do not normally appear at peaceful protests, and that there were no other protests against the film elsewhere in Libya.
"Jihadists will want the world to believe that the attack is just a part of the protests against an amateur film produced in the U.S., which includes crude insults regarding the Prophet Mohammed. They will want the world to think that their actions represent a popular Libyan and wider Muslim reaction; thus, reversing the perception of jihadists being outcasts from their own societies," Quilliam President Noman Benotman said.
But on Thursday, three U.S. officials told CNN that they have seen no evidence the attack was premeditated.
The investigation and quest for justice
The United States is deploying warships and surveillance drones in its hunt for the killers of the four U.S. diplomatic staffers, and a contingent of 50 Marines has arrived to boost the security of Americans in the country.
The drones are expected to gather intelligence that will be turned over to Libyan officials for strikes, the official said.
Two American destroyers are en route to the Libyan coast, U.S. officials told CNN. Both the USS Laboon and USS McFaul are equipped with satellite-guided Tomahawk cruise missiles that can be programmed to hit specific targets.
The move "will give the administration flexibility" in case it opts to take action against targets inside Libya, one senior official said. As of late Wednesday, the McFaul was making a port call on the Mediterranean island of Crete, while the Laboon was outside Gibraltar, a few days away from Libya.
"We will not waver in our commitment to see that justice is done for this terrible act," U.S. President Barack Obama said Wednesday. "And make no mistake, justice will be done."
Obama called Libya's Mohamed Magariaf on Wednesday, thanking the newly elected president of Libya's parliament for his condolences on the deaths of the Americans.
"The two presidents agreed to work closely over the course of this investigation," the White House said in a statement. Obama "reaffirmed our support for Libya's democratic transition, a cause Ambassador Stevens believed in deeply and did so much to advance. He welcomed the election of a new prime minister yesterday to help lead the Libyan government's efforts to improve security, counter extremism, and advance its democracy."
Tuesday's attack took place on the 11th anniversary of the al Qaeda attacks on New York and Washington. But White House National Security Council spokesman Tommy Vietor said assigning any motive for the attack was "premature."
State Department spokeswoman Victoria Nuland said the evaluation of security at the Benghazi, Libya, consulate in advance of the September 11 anniversary was "appropriate for what we knew." She cited a local guard force stationed around the outer perimeters and a "robust" American security presence in the compound.
Libya's response and ties to the United States
Libyan leaders apologized for the attack, with Prime Minister Abdurrahim el-Keib calling it a "cowardly, criminal act."
Obama said that despite the inflammatory movie, the violence was 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."
The United States and Libya have embarked on a new relationship since rebels toppled longtime dictator Moammar Gadhafi last year.
U.S. and NATO warplanes helped the Benghazi-based rebellion against Gadhafi, who was wanted by the International Criminal Court for charges of crimes against humanity before the ruler was killed in October.
The jihadists suspected in Tuesday night's attack "are a very small minority" who are taking advantage of a fledgling democracy, said Ali Suleiman Aujali, the Libyan ambassador to the United States.
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. 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.
How the attack happened
On Tuesday night, protesters were outside the consulate in Benghazi, demonstrating against the film "Innocence of Muslims," which reportedly was made in California by a filmmaker whose identity is unclear.
Eventually, a group of heavily armed militants "infiltrated the march to start chaos," according to Libyan Deputy Interior Minister Wanis al-Sharif.
Initial reports indicate the four-hour assault began around 10 p.m. as gunmen opened fire on the main compound of the U.S. Consulate complex. Within 15 minutes, the gunmen entered the building.
A senior U.S. official said a rocket-propelled grenade set the consulate ablaze. American and Libyan security personnel tried to fight the attackers and the fire.
As the fire spread, three people -- Stevens, Foreign Service information management officer Sean Smith, and a U.S. regional security officer -- were inside a safe room, senior State Department officials said.
Smith was later found dead, apparently of smoke inhalation, officials said. It's unclear how Stevens died.
Glen Doherty, a guard, was also killed at the consulate.
One other American, whose name hasn't been released, was killed, and another two were wounded during a gun battle between security forces and militants at the complex, a senior administration official said.
U.S. Secretary of State Hillary Clinton praised Stevens and his commitment to Libya on Wednesday.
"He arrived on a cargo ship in the port of Benghazi and began building our relationships with Libya's revolutionaries," she said after his death was announced. "He risked his life to stop a tyrant, then gave his life trying to build a better Libya."
Doherty's sister, Katie Quigley, spoke to reporters Thursday outside the family home in Woburn, Massachusetts, outside of Boston.
"Glen lived his life to the fullest," she said. "He was my brother, but if you ask his friends he was their brother as well."
CNN's Jomana Karadsheh, Elise Labott, Suzanne Kelly, Barbara Starr, Nic Robertson, Matt Smith and Michael Martinez contributed to this report.
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