* White House labels assault on consulate "terrorist" attack
* Lawmakers want answers on envoy's death, security
* Probing possible collusion between attackers, guards
By Susan Cornwell and Mark Hosenball
WASHINGTON, Sept 20 (Reuters) - The Obama administration on Thursday described last week's assault on the U.S. Consulate in Benghazi, Libya, as a "terrorist attack" and announced a panel to investigate the events that took the lives of the ambassador and three other Americans.
Secretary of State Hillary Clinton gave lawmakers a classified briefing as more questions were raised in Congress about whether sufficient security was in place before the Sept. 11 attack in which the Americans, including Ambassador Christopher Stevens, died.
Clinton said the investigating panel would be chaired by Thomas Pickering, a retired diplomat who served as U.S. ambassador to Russia, India, Israel, Nigeria, El Salvador, Jordan and at the U.S. Mission to the United Nations.
The White House said it agreed with an assessment made a day earlier by a senior counterterrorism official that the violence in Benghazi was an act of terrorism.
"It is self-evident that what happened in Benghazi was a terrorist attack," White House spokesman Jay Carney told reporters traveling with President Barack Obama. Carney did not go any further in clarifying whether the administration believed the attack was planned.
Some Republicans said they saw a shift in emphasis from the White House's earlier presentation of the violence as a protest outside the Benghazi consulate that got out of control.
Debate over whether militant groups planned the assault or whether the violence resulted from protests against a film insulting to Islam has become U.S. election-year fodder.
"The story now has been changed. There was a planned, premeditated attack," Republican Representative Howard McKeon, chairman of the House Armed Services Committee, told reporters on Capitol Hill.
The investigative panel, whose creation is generally required by law when someone is killed or seriously injured at a U.S. mission abroad, is made up of four people chosen by the secretary of state and the U.S. intelligence community. It is expected to write a report on whether security systems and procedures were adequate, and could recommend improvements.
Its work is separate from an FBI probe of the Benghazi attack, which happened on the 11th anniversary of the Sept. 11 attacks on the United States.
U.S. authorities are investigating possible collusion between the militants who launched the attack and locally hired Libyan personnel guarding the facility, three U.S. officials said on condition of anonymity. So far there is no proof of this, they said.
Lawmakers have demanded answers on how Stevens, a State Department information management officer and two security agents could have died in the incident. Stevens' death marked the first time a U.S. ambassador had been killed in such an attack since 1979.
U.S. embassies in Egypt, Sudan, Tunisia and Yemen have been attacked and U.S. diplomatic facilities in the Middle East and North Africa have been the target of protests sparked by a film made in California that depicts the Prophet Mohammad as a womanizer and a fool.
Appearing at a forum sponsored by Univision and Facebook, and hosted by the University of Miami in Coral Gables, Florida, Obama said the United States would not retreat from the region.
"My message to the presidents of Egypt, Libya, Tunisia and these other countries: we want to be a partner with you, we will work with you and we stand on the side of democracy," he said.
"But democracy is not just an election, it's also are you looking out for minority rights, are you respecting freedom of speech, are you treating women fairly?" he added.
"The one thing we can't do is withdraw from the region. The United States continues to be the one indispensable nation."
Speaking at a news conference before she briefed U.S. lawmakers, Clinton also stressed the importance of U.S. relations with such countries despite questions about whether the United States should continue aid following the protests.
A congressional committee wrote to Clinton on Thursday demanding information about the attack in Benghazi, including all U.S. security analyses and threat assessments before the violence and any documents that clarify whether the attack was spontaneous or premeditated.
"The American people have a right to know the facts about this egregious attack on U.S. sovereign territory," Republican Representative Jason Chaffetz wrote to Clinton, setting an Oct. 4 deadline for her to provide the information.
Adam Smith, the senior Democrat on the House Armed Services Committee, attended part of the briefing Thursday with Clinton and said - as Reuters reported on Wednesday - that the U.S. ambassador to Libya had five security guards with him. Smith said he thought that was an appropriate number.
Asked about possible collusion between Libyans working for the U.S. Consulate in Benghazi and the attackers, Smith said, "There is no evidence of that at this time."