WASHINGTON - A spokesman for President Barack Obama said on Friday officials had no evidence the attack that killed the US ambassador to Libya was pre-planned, an assertion which added to confusion over the incident.

Immediately after the attack on the US consulate in Benghazi, Libya, on Tuesday night, US officials, speaking on condition of anonymity, were quoted widely in the media saying they believed the attack was well-planned and organized.

On Friday, however, President Barack Obama's press secretary, Jay Carney, offered a different version of events. "We do not at this moment have information to suggest or to tell you that would indicate that any of this unrest was pre-planned," Carney told a press briefing.

The confusion over what the US government knows about the attack was compounded by statements on Friday by a leading US senator. Following a briefing by US Defense Secretary Leon Panetta, Carl Levin, the Democrat who chairs the Senate Armed Services Committee, told journalists he understood the attack was planned and premeditated.

Another US official said: "Everything I have seen says this was a highly armed, organized attack. Not a mob reacting to the movie. Whether it was planned or not is another question."

While some of the key facts remain unclear, if it is ultimately determined the attack was planned in advance, that could prove embarrassing to Obama, who is fending off attacks from Republican presidential nominee Mitt Romney on his handling of escalating anti-American violence in the Middle East.

US Ambassador to Libya Christopher Stevens and three other Americans were killed in the violence.

Officials of some agencies directly involved in investigating the Benghazi attack said that, because the FBI has launched a full-scale inquiry, they have been forbidden from publicly commenting on what is being learned.

However, US officials familiar with the incident said the White House assertion that it has no information indicating the violence was planned, while arguably true in a limited context, simplifies what the US government knows.

The sources, who asked for anonymity when discussing sensitive information, said that based on information currently available, most other government officials believe there was at least some planning and organization behind the Benghazi attack.

Apart from anything else, the sources noted, heavy weaponry, including rocket-propelled grenades and mortars, allegedly was used by militants during the course of the attack. Deployment of such weaponry almost certainly would have required some advance organization or planning.

One of the sources said that accumulating evidence suggests that organized militants, with some modicum of planning, may well have taken advantage of what started out as a spontaneous mob demonstration protesting a short film, made in California, which lampooned the Prophet Mohammed.

At the same time, he said, hard evidence so far is lacking that the planning behind the attack began long in advance of the mob demonstration.

Asked on Friday whether the US had any advance intelligence about a possible attack, Carney insisted: "We were not aware of any actionable intelligence indicating that an attack on the US mission in Benghazi was planned or imminent."

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