The State Department is opening a new inquiry into the attack on a diplomatic post in Libya that killed four Americans, Secretary of State Hillary Rodham Clinton said Thursday.

Clinton announced the inquiry the same day she and senior defense, intelligence and law enforcement officials briefed Congress about what the Obama administration has learned about the attack in Benghazi during the past week.

The classified presentations to both houses of Congress included a look at U.S. intelligence assessments ahead of the Sept. 11 attack in Libya and the FBI’s efforts to identify suspects.

The administration presentation was a careful examination of what is known about the perpetrators of the attack, intelligence about possible threats related to the attack and the State Department’s security inquiry, a person familiar with the briefing said. The session didn’t offer a firm conclusion about what group or groups were responsible, the person said.

“They are really doing a very thorough forensic analysis. They are being very judicious and cautious,” the person said.

The person requested anonymity to describe the broad outlines of the classified session.

The investigation has not filled in all the gaps in the confusing timeline of the hours-long attack, the person said. It still is not clear exactly when Stevens died, and other details are hazy. No cause of death was given for the four Americans.

Speaking earlier with reporters, Clinton said the briefing would cover “our security posture before and during the event and the steps we have taken since to do everything we can with host governments to protect our people and our embassies and consulates.”

A senior U.S. official said earlier in the day that Director of National Intelligence James R. Clapper was expected to tell members of Congress that he does not consider the attack in Benghazi an indication of a long-planned operation, a senior U.S. official said.

Clapper was also expected to tell Congress that there could be multiple extremist elements involved or linked to the attack but that reports claiming involvement by particular people probably are inaccurate, the official said. The official spoke on the condition of anonymity to describe the agenda for the closed-door sessions.

The White House has labeled the Benghazi incident a terrorist attack. Asked if the attack was carried out by al-Qaeda, President Obama said, “We don’t know yet.”

Speaking at a candidate forum on Spanish-language network Univision, Obama said groups affiliated with al-Qaeda “have not gone away” and remain dangerous.

More than a week after the lethal attack, conflicting accounts continue to emerge both in Washington and Benghazi about whether the assault was spontaneous or premeditated. The Obama administration insists that so far evidence indicates local extremists hijacked a peaceful demonstration, as has been suggested in Benghazi.

A spokesman for Ansar al-Sharia, one of the militant groups blamed for the assault, acknowledged that members of the group were among many militia members who attended the protest over an anti-Muslim video, but said the attack was carried out by others who arrived at the consulate and began to disperse the crowd.

“It’s like a culture here,” said the spokesman, Hany Mansouri. “You carry your weapons. Everyone has Kalashnikovs, RPGs [rocket-propelled grenades] and small guns, at least.”

But a former member of Libya’s Transitional National Council with close knowledge of the investigation there said that there are signs the assault was planned, the same position taken by the Libyan government.

The former TNC member, who spoke on the condition of anonymity because he is not authorized to speak to the press, pointed to a successful mortar strike on a safe house outside the consulate as evidence of advance planning. He said the attackers needed experience using mortars and knowledge of the location of the building.

Sen. Susan Collins (R-Maine) said at a Senate hearing on Wednesday that, after briefings, she has concluded that the attack was planned. “I just don’t think that people come to protests equipped with RPGs and other heavy weapons,” she said.

The inquiry announced by Clinton will be carried out by an independent four-member panel chaired by retired diplomat Thomas Pickering. The panel, required by law, will look at whether security procedures were adequate at the U.S. Consulate in Benghazi and make recommendations to the secretary of state. Pickering was once the boss of U.S. Ambassador J. Christopher Stevens, who was killed in the Benghazi attack last week, along with another diplomat and two security personnel.

Speaking at a news conference with Indonesian Foreign Minister Marty Natalegawa, Clinton also said she would argue to Congress that the United States must remain engaged in Muslim-majority nations that have seen more than a week of anti-American protests.

Hauslohner reported from Benghazi.