Secretary of State Hillary Clinton Thursday said that she was forming a panel to investigate the attack on the U.S. Consulate in Benghazi that led to the killing of U.S. Ambassador to Libya Chris Stevens and three other Americans.

She will hold closed-door classified briefings with members of the House and Senate separately on the attacks after discussing security situation at the consulate before and during the Sept. 11 rocket attack.

She said she would be joined by Director of National Intelligence James Clapper and Deputy Defense Secretary Ashton Carter, reported the Associated Press.

Retired diplomat Thomas Pickering will chair the independent, bipartisan panel which has been appointed in line with federal law, Clinton said. Pickering has previously served as the U.S. ambassador to Russia, India, Israel, Nigeria, El Salvador, Jordan and at the U.S. Mission to the U.N.

The panel's inquiry is separate from an FBI probe of the Benghazi attack. An FBI team reached Tripoli Wednesday to join the investigation by the local officials.

Meanwhile, U.S. officials appeared less certain about what transpired in Benghazi on the night of the deadly attack, though they maintained that a spontaneous protest over an anti-Islamic movie led to the attack, CNN reported.

"Soon after that spontaneous protest began outside of our consulate in Benghazi, we believe that it looks like extremist elements, individuals, joined in that effort with heavy weapons of the sort that are, unfortunately, readily now available in Libya post- revolution. And that it spun from there into something much, much more violent,” U.S. Ambassador to the U.N. Susan Rice was quoted saying on CBS show “Face the Nation.”

However, Republican senators like John McCain have contested this theory saying that it was unlikely to be a spontaneous attack given the amount of weaponry involved.

National Counterterrorism Center Director Matthew Olsen told a Senate committee Wednesday that the evidence gathered so far didn’t indicate prior planning involved in the attack.

"The best information we have now, the facts we have now indicate this was an opportunistic attack on our embassy. The attack began and evolved and escalated over several hours," Olsen was quoted saying by CNN. "It appears that individuals, who were certainly well armed, seized on the opportunity that was presented as the events unfolded that evening into the morning hours of September 12th."

However, he called the incident a “terrorist attack” adding, “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.”

“We are looking at indications that individuals involved in the attack may have had connections to al-Qaeda or its affiliates, in particular al-Qaeda in Islamic Maghreb,” Olsen said in response to the question from Senator Joe Lieberman.

White House press secretary Jay Carney on Thursday said it was “self-evident that what happened in Benghazi was a terrorist attack.”

The Obama administration was playing down the possibility of the Libyan security personnel hired by the consulate helping the attackers, Reuters reported. However, investigators seriously consider the question of whether the perpetrators had inside information or help to carry out the attack which coincided with the 11th anniversary of the Sept.11 attacks on the U.S.