An official with the National Counterterrorism Center officially described the September 11 assault on the US consulate in Benghazi, which killed four Americans including Ambassador Christopher Stevens, as a terrorist attack, though he hedged by saying it was an “unplanned” one.

“I would say they were killed in the course of a terrorist attack,” said Matthew Olsen, director of the National Counterterrorism Center, told the Senate Committee on Homeland Security and Governmental Affairs.

It was the first time that a senior administration official had said the attack was not the result of a demonstration over an anti-Islam video that has been cited as the spark for protests in dozens of countries over the past week. “The picture that is emerging is one where a number of different individuals were involved,” Olsen said.

His comments came as significant questions persisted about the consulate’s security: The attack took place on the anniversary of the Sept. 11, 2001, terror attacks on the U.S.; Americans were known to be under threat, and Benghazi had experienced a string of attacks on foreign targets during the summer. Moreover, Libya remains plagued by armed groups nearly a year after the U.S.-backed ouster of the late dictator Moammar Gadhafi. Yet the facility was primarily defended by local guards who may have been complicit.

Olsen described the event as an “opportunistic attack,” basically alleging that militant groups took advantage of the turmoil at the consulate, caused in part by the release of an anti-Islam video that sparked protests at US embassies throughout the world. The attack “evolved” over several hours, he said, and included “a number of different elements” operating in Libya. Olsen held out the possibility that al Qaeda or its affiliates were involved. He added that the US had no specific intelligence regarding an attack beforehand.

The claim of an unplanned attack that took advantage of circumstances does not totally bear out. McClatchy reported eyewitness testimony last week from a guard who said that there were no protests at all – “there wasn’t a single ant outside” – until 125 armed militia men attacked the consulate. That contradicts the claim from Olsen that the attack evolved over several hours. The guard also claimed that the attackers carried a banner from Ansar al Sharia, a militia group in the country.

In addition, the attack on the safe house where US personnel were eventually taken suggests more of a coordinated, premeditated assault than the US alleges. And now this news emerges.

In the months leading up to his death, Chris Stevens, the U.S. ambassador to Libya, worried about constant security threats in Benghazi and mentioned that his name was on an al Qaeda hit list, a source familiar with his thinking told CNN.

Stevens spoke about a rise in Islamic extremism and al Qaeda’s growing presence in Libya, the source said.

Other sources have contradicted the US and alleged that the attack was planned, including the leader of the interim government in Libya. Even Joe Lieberman and Susan Collins, the co-chairs of the hearing, stated pretty flatly that the attack was premeditated. Collins referred to the video from current al Qaeda leader Ayman al-Zawahiri, calling on Libyans to attack Americans in response to a drone strike on a former #2 in the terrorist organization, a Libyan.

Meanwhile, three security officials in Libya have quit in the last week in response to the security breakdown in Benghazi.

UPDATE: Glenn Greenwald does his usual thorough job with this story.