A brewing conflict between Congress and the Obama administration broke into the open Thursday as several lawmakers were critical about a briefing on the Sept. 11 anniversary attack on U.S. diplomats in Libya, which the administration had said was a spontaneous response to an anti-Islam video.

Secretary of State Hillary Rodham Clinton and other administration officials held a series of closed-door briefings for members of both houses over how to conduct an independent investigation of the lethal assault on the U.S. Consulate in Benghazi, which the administration for the first time Thursday conceded was a “terrorist attack.”

But after the briefings, Republicans were openly skeptical about the information they had just been given, and even some Democrats acknowledged that the administration’s initial claims don’t seem to hold up. The attack last week took the lives of four Americans.

“There’s increasing amount of evidence that this was a coordinated attack by terrorists,” said Sen. Marco Rubio, Florida Republican and a member of the Foreign Relations Committee. “The movie wasn’t the catalyst for this. The catalyst was radical Islamic extremists that wanted to attack the United States and saw an opportunity to do it in Benghazi.”

Sen. John McCain, Arizona Republican, was openly dismissive of the administration. “I’m stunned that they thought this was some kind of spontaneous demonstration,” he told reporters.

Rep. Adam Smith, Washington Democrat and member of the House Armed Services Committee, said that while heavy weapons such as mortars weren’t used until several hours after the fighting started, it was “clearly a terrorist attack,” albeit one that more resembled “an armed gang that seized an opportunity” than a perfectly executed plot like the original Sept. 11 terrorist attacks.

The developments came on another day of acute awareness among U.S. officials of growing anger in the Muslim world, as the State Department began running public service messages on Pakistani TV distancing the U.S. from the anti-Islam film that has enraged the Muslim world, and has opened investigations into the shortcomings of security for its diplomats.

On Thursday, the State Department said that embassy officials in Islamabad have paid roughly $70,000 to buy advertising on seven Pakistani TV stations to air a public service message aimed at dampening perceptions that the “Innocence of Muslims” Internet video that denigrates Islam’s Prophet Muhammad was made by the U.S. government.

Department officials said the message features a recent news clipof Mrs. Clinton saying, “Let me state very clearly that the United States has absolutely nothing to do with this video. We absolutely reject its contents.”

Dubbed in the Urdu language of Pakistan, the message also features a recent clip of President Obama saying that “since our founding, the United States has been a nation of respect – that respects all faiths” and that “we reject all efforts to denigrate the religious beliefs of others.”

Also Thursday, the White House reversed course and conceded for the first time that the assault that killed the Americans in Benghazi was in fact a terrorist attack.

“It is, I think, self-evident that what happened in Benghazi was a terrorist attack,” White House spokesman Jay Carney told reporters traveling Thursday with President Obama aboard Air Force One. “Our embassy was attacked violently, and the result was four deaths of American officials. So, again, that’s self-evident.”

The remarks signal a shift in the narrative set by the Obama administration, which has resisted framing the attack as premeditated.

For more than a week since the attack, the administration had characterized it as the result of a protest triggered by anger toward “Innocence of Muslims.” Skeptics have pointed out that some in the crowd were using heavy weapons, such as rocket-propelled grenade launchers, in the assault.

State and Congress

Before heading into one of the meetings on Capitol Hill on Thursday, Mrs. Clinton announced the formation of an official State Department review panel, which will examine whether security was adequate at the time of the Benghazi attack.

She told reporters that she planned to brief lawmakers on the “security posture before and during” the Benghazi attack along with “steps we have taken since to do everything we can with host governments to protect our people and our embassies and consulates.”

Mr. Clinton also said she has “absolutely no information or reason to believe that there’s any basis” for reports that American Ambassador J. Christopher Stevens was operating under the fear that he was on an al Qaeda hit list before he was killed last week in Libya.

The panel will be headed by retired Ambassador Thomas R. Pickering, Mrs. Clinton said.

It remains to be seen how U.S. lawmakers will respond to Mrs. Clinton’s move to create an official panel to examine security questions that have emerged out of last week’s attack in Benghazi.

Last week, the chairman and the ranking Republican of the Senate Homeland Security & Government Affairs Committee sent a letter to the State Department inspector general asking for a thorough, independent investigation of security at diplomatic outposts around the world.

“The State Department Deputy IG agrees with me that an independent assessment is ultimately necessary,” Sen. Susan M. Collins of Maine, the ranking Republican, said Thursday in an email.

The office of the inspector general did not return calls seeking comment, but a State Department official said, without elaborating, that Mrs. Collins had misconstrued the inspector general’s communication to her.

However, on Thursday night, a spokesmanfor Mrs. Collinstold The Washington Times that the State Department’s inspector general investigation “would be separate” from that being directed by Mrs. Clinton.

Pakistan, Indonesia, Libya

In Los Angeles, a Superior Court judge refused to order the social-media company YouTube, which is owned by the Internet search engine Google, to remove a 14-minute trailer of the crudely made anti-Muslim film. The judge rejected a plea from an actress who had argued that she had been duped into appearing in the film.

The film has been blamed for sparking widespread anti-Western demonstrations that saw American flags desecrated and U.S. Embassy walls breached in several Middle Eastern cities over the past week.

Most of the demonstrations have died down during recent days. But not in Pakistan, where anti-American sentiment was already running high, and U.S. officials are now raising fresh concerns about ongoing and potentially volatile protests near the U.S. Embassy in the capital city of Islamabad.

The U.S. Embassy in Pakistan remained open Thursday, but officials said other diplomatic posts were closed in the nation, as well as in other parts of the world where demonstrations have surged.

Mrs. Clinton, who appeared at the State Department after bilateral talks with Indonesian Foreign Minister R.M. Marty Natalegawa, said U.S. facilities in Indonesia – home to the world’s largest population of Muslims – would be temporarily closed Friday, the Islamic holy day.

It is not unusual for U.S. diplomats to keep business hours in line with those of their host nations and, in the Muslim world, Friday is typically treated like a Sunday in the United States.

But Mrs. Clinton said U.S. officials also “want to be sure that law enforcement in Indonesia has the ability to do what it needs to do to make sure that there is no disruption of civil order and security.”

With regard to the developments involving Libya, Deputy Secretary of State William J. Burns appeared Thursday in the capital city of Tripoli for a ceremony honoring Mr. Stevens and the three other Americans killed last week.

“We have lost four wonderful colleagues,” Mr. Burns told Libyans gathered for the ceremony. “We have lost a brilliant ambassador, full of courage and skill and passionate determination to help Libyans, to help all of you, realize the promise of your revolution, to make a reality of a free Libya.”

Mr. Burns added that “neither Chris Stevens nor our three other fallen colleagues were naive. They were not blind in their optimism. They knew the troubles Libyans faced and the risks they had to endure.”

Al Qaeda speculation

On Wednesday, Matthew Olsen, director of the National Counterterrorism Center, told the Senate Foreign Affairs Committee that the Benghazi assault was “an opportunistic attack,” although he repeated the Obama administration’s assertion that it was not a premeditated assault.

Mr. Olsen said those involved in the violent assault appeared to have come from several militant groups, including affiliates of al Qaeda.

Intelligence officials Thursday dismissed a news report that had suggested the attack may have been masterminded by a man who was once held by U.S. authorities at the U.S. Naval Base Guantanamo Bay, Cuba.

“It is safe to assume that any significant extremist in Eastern Libya is going to be under a lot of scrutiny right now,” a U.S. intelligence official told The Times. But, the official added, “Any suggestion that a leading suspect or ‘mastermind’ of the attack has been identified at this point is premature.”

Accountability review

Federal law requires the secretary of state to convene an “accountability review board” to conduct an examination in the event of an attack on a U.S. embassy or diplomatic post like the one that occurred in Benghazi.

Mrs. Collins asserted Wednesday that it is “imperative that a nonpolitical, no-holds-barred examination be conducted” and that it must be conducted by “an independent entity like the inspector general.”

But accountability review boards formed in the past have tended to include sobering and nonpolitical revelations about the security posture of U.S. diplomats.

Such revelations have generally taken about six months to come to the fore and have a tendency of being ignored by officials after the initial media frenzy surrounding an attack like the one in Benghazi has subsided.

The best example of incendiary findings dredged up by a past accountability review board may be those from the board that examined security questions that arose during the aftermath of the 1998 terrorist bombings that killed 258 people at the U.S. embassies in Kenya and Tanzania.

A January 1999 letter from officials who headed the board to then-Secretary of State Madeleine K. Albright highlighted the “inadequacy of resources to provide security against terrorist attacks” prior to the Kenya and Tanzania incidents, as well as the relatively “low priority accorded security concerns throughout the U.S. government.”

The letter was written just over two years prior to the security failures that allowed terrorists to carry out the horrific attacks of September 11, 2001, on the United States.

The coming months will tell whether the board that Mrs. Clinton has chosen to be lead by Mr. Pickering yields anything truly insightful about the posture of U.S. diplomats last week.

Mr. Pickering, 81, is among the most decorated former U.S. diplomats. But he has been largely out of the spotlight since retiring from the State Department in 1996 after serving for four years as U.S. ambassador to Russia. He had previously served as ambassador to the United Nations, India, Israel, El Salvador, Nigeria and Jordan.

• Shaun Waterman and Dave Boyer contributed to this report.