Editor's note: Read a version of this story in Arabic
Washington (CNN) -- The nation's counterterrorism chief told Congress on Wednesday the assault on a U.S. diplomatic mission in Libya that killed Ambassador Christopher Stevens and three other Americans was a terrorist attack.
But Matthew Olsen said at a Senate hearing the best information so far indicates that armed extremists did not plan in advance to assault the Benghazi consulate last Tuesday, but took advantage of an opportunity to do so during a demonstration over an anti-Muslim film.
Olsen said the investigation continues and facts are being developed. But he said it "appears that individuals who were certainly well armed seized on the opportunity presented as the events unfolded that evening and into the morning of September 12.
"We do know that a number of militants in area, as I mentioned, are well armed and maintain those arms. What we don't have at this point is specific intelligence that there was a significant advanced planning or coordination for this attack," he said.
That point was stressed as well by White House spokesman Jay Carney on Wednesday.
"It is a fact that there are in post-revolution, post-war Libya armed groups, there are bad actors, hostile to the government, hostile to the West, hostile to the United States and as has been the case in other countries in the region, it is certainly conceivable that these groups take advantage of and exploit situations that develop when they develop to protest against or attack either westerners, Americans, western sites or American sites," Carney said.
Some senators on the Homeland Security Committee did not agree with the suggestion that the attack was spontaneous.
Sen. Susan Collins, the Republican vice chairman of the panel from Maine, is convinced the incident was a premeditated, pre-planned attack tied to the anniversary of the September 11, 2001, al Qaeda attacks on the United States.
"The attack in Benghazi was not a black swan, but rather an attack that should have been anticipated based on previous attacks against western targets, the proliferation of dangerous weapons, the presence of al Qaeda in that country and the overall threat environment," Collins said. "I just don't think that people come to protest equipped with RPGs (rocket propelled grenades) and other heavy weapons."
Committee Chairman Joseph Lieberman, a Connecticut Independent, said he tended to agree with Collins, but would wait for the investigation to conclude.
FBI agents arrived in Libya on Tuesday to join other U.S. and Libyan officials investigating the attack.
FBI Associate Deputy Director Kevin Perkins told the committee it was an open investigation.
"We are conducting interviews, gathering evidence and trying to sort out the facts, working with our partners both from a criminal stand point as well as in the intelligence community to try and determine exactly what took place on the ground that evening," he said.
As for who was responsible, Olsen said there are a number of militant groups in eastern Libya and investigators are looking to see if there are any connections with al Qaeda, in particular the affiliate al Qaeda in the Islamic Maghreb.
Another point of contention at the hearing was whether the consulate was adequately protected.
Collins said it was clear the security situation was deteriorating in Benghazi, citing four previous attacks against western targets, including the U.S. consulate, since June.
Collins said that she was "stunned and appalled" to find out U.S. Marines were not guarding the mission and that foreign nationals were mostly responsible for security.
Olsen said it is common for a host country to help guard U.S. diplomatic posts and the threats in Libya were well known. But he said there was no specific intelligence that the Benghazi post faced an imminent attack.
Separately, Sen. John McCain, an Arizona Republican, told reporters on Capitol Hill on Wednesday that he had information there had been warnings.
"They had previous warnings of pending attacks - in every way," McCain said.
He said that he was not getting adequate information from U.S. government officials about what happened in Libya and was relying on alternate sources for information.
"Other sources of information was that security was very much lacking, now whether that is absolutely true or not I cannot say yet," McCain said, though he did not elaborate on where he got the information.
"We have information that there was -- and I do not know if it's totally accurate -- that there were previous warnings of attacks, there was a very extremist militia and that there was very lax security at the consulate, but I have no official corroboration," McCain said.
CNN's Arwa Damon reported this week that, according to Libyan military officials, just three days before the attacks, the Libyans had a meeting with senior employees from the consulate where they were talking about this rising threat against western interests.
The officials said the meeting highlighted the point that the Libyan government could not control militias.
Several U.S. officials told CNN that although there was a general concern about extremism in eastern Libya, there were no warnings that the Benghazi post was a specific target.
The United States did beef up security after a failed bomb attempt in June and a pre-September 11 review of base security deemed the post to be adequately secured.
"We had no actionable intelligence that an attack on our post in Benghazi was planned or imminent," Secretary of State Hillary Clinton said on Tuesday.
Deirdre Walsh and Elise Labott contributed to this report.