This month's deadly assault on the U.S. Consulate in eastern Libya was clearly a planned attack by terrorists, U.S. Defense Secretary Leon Panetta said Thursday.

"As we determined the details of what took place there and how that attack took place, it became clear that there were terrorists who planned that attack," Panetta said.

Speaking to reporters at the Pentagon, Panetta said investigators had not yet determined exactly what terrorists were involved in the attack on the U.S. Consulate in Benghazi.

"There's a lot of different kinds of terrorism in that part of the world," he said.

It's the administration's latest assessment of the September 11 consulate attack that killed the U.S. ambassador to Libya and three other Americans. The incident fueled increased global scrutiny of the North African nation and increased political sparring in the United States over the investigation into who was behind it.

The FBI is investigating the attack, although its agents are in Tripoli and have yet to access the consulate, which is still unsecured, according to senior law enforcement sources. Libyan officials have said they have brought in dozens of people for questioning in connection with the attack.

Conflicting assessments

In the days after the assault on the Benghazi consulate, U.S. administration officials offered conflicting assessments of what led to the fatal security breach. Some top officials said the violence erupted spontaneously amid a large protest about a U.S.-made film that mocked the Muslim Prophet Mohammed.

Administration, intelligence officials and lawmakers have disagreed about whether the violence was the result of a mob gone awry, a planned terror attack or a combination of the two.

On September 14, White House spokesman Jay Carney said U.S. authorities had "no actionable intelligence" indicating the attack "was planned or imminent."

Two days later, Susan Rice, U.S. ambassador to the United Nations, said U.S. officials believed extremists carried out the attack after a spontaneous protest began outside the building.

"We do not have information at present that leads us to conclude that this was premeditated or preplanned," she said.

On September 19, Matthew Olsen, the nation's counterterrorism chief, told senators it had been a terrorist attack. The next day, Carney also said it was "self-evident that what happened in Benghazi was a terrorist attack."

The administration's response has drawn sharp criticism from Republican lawmakers, who have argued that it mischaracterized the attacks and gave unnecessary credence to the film.

"They were either incredibly naïve or willfully deceiving the American people. I don't know which," Republican Sen. John McCain told CNN's "Anderson Cooper 360" on Thursday.

Asked when he had concluded that terrorists were behind the consulate attack, Panetta said, "It took a while to really get some of the feedback from what exactly happened at that location."

An al Qaeda link?

A senior U.S. official told CNN that there was evidence within 24 hours of the attack that suggested it was the work of extremists either affiliated with al Qaeda groups or inspired by them.

"We started to get a strong sense of it," the official said. He declined to be identified due to the sensitivity of the information.

The efforts by al Qaeda, especially the Mali-based al Qaeda in the Islamic Maghreb, to extend its reach into Libya and elsewhere has been of concern to the United States, something that Secretary of State Hillary Clinton mentioned on Wednesday.