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

Immediately after the attack on the U.S. consulate in Benghazi, Libya, on Tuesday night, U.S. 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 preplanned," Carney told a press briefing.

The confusion over what the U.S. government knows about the attack was compounded by statements on Friday by a leading U.S. senator. Following a briefing by U.S. 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 U.S. 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.

U.S. 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, U.S. 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 U.S. government knows.

HEAVY WEAPONRY USED

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.

A senior administration official with access to the most up-to-date intelligence defended Carney's account.

"The currently available information suggests that the demonstrations in Benghazi were spontaneous, inspired by protests at the U.S. Embassy in Cairo, and evolved into the assault against the consulate," said the official, speaking on condition of anonymity.

The official said there was no indication at this point that the Benghazi attack was pre-meditated well in advance, but that there are plenty of weapons in public hands and militants were mixed in among the mobs.

"We do know that Islamic extremists participated in some of the demonstrations but not necessarily one group with top-down control," the official said.

In the immediate aftermath of the attack, some experts advanced the theory that Libyan militants tied to Al Qaeda began planning some time ago to attack the consulate on the anniversary of the September 11 2001 attacks on New York and Washington D.C.

A related theory is that militants pre-planned the attack to follow the release of a video in which Ayman al-Zawahiri, the leader of what remains of Al Qaeda's core group, condemned the killing of one of his Libyan deputies in a U.S. drone attack and called for retaliation.

U.S. officials told Reuters on Thursday that although an Arabic satellite-TV talk show aired parts of the anti-Islamic film last Saturday, the broadcast did not prompt a major upgrade in security precautions.

Two U.S. officials said on Friday that after becoming aware of the broadcast, Washington did send some kind of low-key but "specific" advisory about it to U.S. diplomatic posts. But the message was not so alarming as to lead to a major upgrade in security for a possible emergency.

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