WASHINGTON — The White House for the first time Thursday described the Sept. 11 assault on the U.S. Consulate in Libya as a terrorist attack that may have involved militants linked to Al Qaeda, but it added that no intelligence yet shows it was planned in advance.

The new evaluation came as congressional committees met in closed session to press Secretary of State Hillary Rodham Clinton, along with top intelligence and law enforcement officials, on whether the diplomatic outpost was adequately protected by a force of mostly Libyan guards.

Libyan officials allowed FBI investigators to visit the burned-out compound only early this week, officials said, a delay that could hamper the team in gathering evidence and interviewing witnesses.

Jay Carney, the White House press secretary, said some of the heavily armed men who stormed the consulate in Benghazi and killed U.S. Ambassador J. Christopher Stevens and three other Americans “may have had connections” to Al Qaeda in the Islamic Maghreb, an offshoot of the terrorist network that is active in eastern Libya.

“It is, I think, self-evident that what happened in Benghazi was a terrorist attack,” Carney told reporters aboard Air Force One. He said U.S. authorities are looking into the “possible participation” of members of Al Qaeda or its affiliates.

Asked later about the Libya attack during a town hall meeting in Coral Gables, Fla., President Obama appeared to fall back on the administration’s earlier description of the assault — that it was sparked by anger over an amateur film, made in California and posted on the Internet, that ridiculed the Muslim prophet Muhammad.

“I don’t want to speak to something until we have all the information,” Obama said. “The natural protests that arose because of the outrage over the video were used as an excuse by extremists to see if they can also directly harm U.S. interests.”

The White House is sensitive to allegations of a security lapse or intelligence failure as Obama argues on the campaign trail that his policies have severely weakened Al Qaeda and reduced the threat of a terrorist attack.

But congressional Republicans have disputed the administration’s initial descriptions of the Benghazi attack as a protest against the video, which sparked anti-American riots and protests in more than 20 countries, from Tunisia to Indonesia.

Republicans and Libyan officials noted that the attackers in Benghazi fired rocket-propelled grenades and other heavy weapons, attacking both the consulate and a safe house where Americans had gathered more than a mile away.

They also cited the security vacuum around Benghazi, Libya’s second-largest city, since the uprising that toppled Libyan leader Moammar Kadafi last year, and a recent series of attacks on Western targets in the area by extremist groups.

Carney said the late-night attack was the “result of opportunism” rather than a planned operation. He echoed comments by the director of the National Counterterrorism Center, Matthew Olsen, who testified at a Senate committee on Wednesday.

“It appears that some well-armed militants seized on the opportunity as the events unfolded that evening,” Carney said. “We do not have specific intelligence that there was significant advanced planning or coordination for this attack.”

Sen. Susan Collins (R-Maine), who had repeatedly challenged Olsen at the hearing, said Thursday that she welcomed the White House shift.

“The fact that the administration is now correctly labeling this a terrorist attack shows me that they are going to investigate and attempt to hold those responsible accountable,” Collins, the ranking Republican on the Senate Committee on Homeland Security and Governmental Affairs, said in a telephone interview

But Collins said she still had concerns about the “woefully inadequate” security at the consulate. Libyan officials have said some of the Libyan guards at the compound may have aided the attackers.

“Given the instability in Benghazi, the previous attack on Western targets — including our consulate —– and the fact that there was an Al Qaeda presence and the city was awash in weapons, it seems to me unwise that our security was so dependent on Libyan guards,” Collins said.

Polls show Obama has strong public support for his handling of national security, a rarity for a Democrat. Although GOP presidential nominee Mitt Romney’s initial criticism of Obama was widely viewed as a misstep, any indications of a terrorist attack may blunt the president’s advantage.

“Now we find out — 10 days later — that Al Qaeda was involved?” asked Sen. Pat Roberts (R-Kan.).

Romney’s campaign released a cautious statement.

“Gov. Romney believes our immediate priority in Libya is to track down and bring to justice those terrorists who brutally murdered our diplomats,” campaign spokesman Ryan Williams said. “The attack is a clear reminder that terrorists — particularly those linked to Al Qaeda — remain a grave threat and one that is growing in North Africa. Gov. Romney believes we must remain vigilant and resolute in our determination to defeat terrorists around the world.”

shashank.bengali@latimes.com

kathleen.hennessey@latimes.com

Times staff writer Ken Dilanian in Washington contributed to this report.