"I would say yes. They were killed in the course of a terrorist attack on our embassy," said Matthew Olsen, Director of the US National Counterterrorism Center.
During a Congressional hearing, Olsen said investigation into the attack is ongoing and facts are being developed continually.
"The best information we have now, the facts that we have now, indicate that this was an opportunistic attack on our embassy. The attack began and evolved and escalated over several hours," Olsen said.
"The well-armed attackers seized on the opportunity presented, as the event unfolded that evening into the morning hours of September 12. We do know that a number of militants in the area are well armed and maintain those arms," he said.
Over the weekend, the Libyan Prime Minister said that his government arrested 50 people for the attack and many of them were foreigners, believed to be part of the Al Qaeda in Arabian Peninsula.
"What we don't have at this point is specific intelligence that there was significant advanced planning or coordination for this attack. Again, we're still developing facts and still looking for any indications of substantial advanced planning. We just haven't seen that at this point," Olsen said.
A number of different elements, including individuals connected to militant groups prevalent in eastern Libya, were involved in the attack and the probe is currently focused on who was responsible for this attack, he said.
"We are looking at indications that individuals involved in the attack may have had connections to the Al Qaeda or its affiliates, in particular the Al Qaeda in Islamic Maghreb," Olsen said in response to the question from Senator Lieberman.
Federal Bureau of Investigation Associate Deputy Director Kevin Perkins told lawmakers that the agency, which is investigating the Benghazi attack has kept its probe open.
"We have an open investigation at this time. We have a significant number of FBI agents, analysts and various support employees assigned to this matter. We are working with our partners, both from a criminal standpoint as well as in the intelligence community, to try to determine exactly what took place that evening," he said.