Fresh claims that U.S. intelligence officials knew practically from the start that the attack on the U.S. Consulate in Benghazi was terrorism possibly tied to Al Qaeda have lawmakers alleging they were misled and questioning whether the administration has something to hide.

"This is turning into something not short of Benghazi-gate," Sen. Bob Corker, R-Tenn., told Fox News, adding he's "very suspicious" about the way the administration has handled this.

Intelligence sources told Fox News on Thursday that U.S. intelligence officials knew within 24 hours of the assault that it was a terrorist attack and suspected Al Qaeda-tied elements were involved.

The account sharply conflicted with claims by U.S. Ambassador to the United Nations Susan Rice on the Sunday after the attack that the administration believed the strike was a "spontaneous" event triggered by protests in Egypt over an anti-Islam film.

"The best information and the best assessment we have today is that in fact this was not a preplanned, premeditated attack -- that what happened initially was it was a spontaneous reaction to what had just transpired in Cairo as a consequence of the video," Rice said on "Fox News Sunday" at the time.

More On This... US officials knew Libya attack was terrorism within 24 hours, sources confirm

Rep. Mac Thornberry, R-Texas, a member of the House intelligence committee, said that's consistent with what lawmakers were told in briefings.

"If there was information a day after that was to the contrary, I think Congress was misled," Thornberry told Fox News. "But again, it's even more serious than that. It means that we have a real problem in not being able to face up to the national security challenges our country faces."

Corker also said a briefing he and his colleagues received was "worthless," and he demanded "answers" about the changing story.

"This has now turned into a very bipartisan concern," he said. "There has to be something that they're trying to hide or cover up. ... This is just not the norm. This is way out of the norm, what is happening in this case."

Democratic and Republican lawmakers on the Senate Foreign Relations Committee, on which Corker sits, sent a letter to the State Department Thursday asking a string of new questions about security at U.S. diplomatic posts.

President Obama's aides have denied any attempt to cover things up. "No one either intentionally or unintentionally misled anyone involved in this," campaign adviser Robert Gibbs said on "Fox News Sunday." "No one wants to get to the bottom of this more than we do."

Curiously, Obama referred to "acts of terror" in his first public remarks about the attack. But from there, administration officials went on to blame the anti-Islam film.

Rice was the most explicit in that explanation, insisting in those Sunday shows that the attack was not pre-planned and was tied to the film. Obama still has not publicly and specifically described the Benghazi attack as terrorism.

But top administration officials have gradually walked back Rice's version of events. Defense Secretary Leon Panetta was the latest Thursday to declare: "It was a terrorist attack." Asked how long it took to make that determination, he said it "took a while to get some of the feedback from what exactly happened at that location."

Secretary of State Hillary Clinton reportedly suggested Wednesday to foreign leaders visiting the United Nations summit in New York that the Al Qaeda affiliate in North Africa was involved.

She was referring to Al Qaeda in the Islamic Maghreb. That's the group that, according to senior officials, the intelligence community suspected by Sept. 12, the day after the attack, of being involved -- along with the militant Ansar al-Shariah.

Officials also confirmed they had several "intelligence assets" on the ground to track Ansar al-Shariha and Al Qaeda in the Islamic Maghreb. They said the attack was labeled as terrorism from the start in order to free up certain resources. Specifically, it was labeled under the "Authorization for the Use of Military Force Against Terrorists" category, created by a post-9/11 congressional action.

Clinton earlier this week called the attack terrorism, two weeks after the fact. White House Press Secretary Jay Carney also said that Obama now believes it was terrorism as well.

Yet a congressional source told Fox News that CIA Director David Petraeus, during a briefing with members of the House Intelligence Committee three days after the attack, also espoused the view that Benghazi was an out-of-control demonstration prompted by the YouTube video. According to the source, this was "shocking" to some members who were present and saw the same intelligence pointing toward a terrorist attack.

In addition, sources confirm that FBI agents have not yet arrived in Benghazi in the aftermath of the attack. Four Americans, including U.S. Ambassador Christopher Stevens, were killed in the assault.

One intelligence official clarified to Fox News that there was not a "definitive" lead on who might have been responsible for the Libya attacks in the immediate aftermath, though officials had an idea of the suspects.

"It's inaccurate to suggest that within the first 24 hours there was a definitive calling card and home address for the perpetrators of the Benghazi attack. Potential suspects and data points emerge early on, but it still takes time to be certain who is responsible," the official said.

Fox News' Bret Baier and Catherine Herridge contributed to this report.