WASHINGTON,DC (CNSNews.com) - As more senior administration officials use the word "terrorism" in describing the deadly September 11 attack on the U.S. consulate in Libya, Ambassador to the U.N. Susan Rice's repeated insistence five days later that the incident was a "spontaneous reaction" to an obscure anti-Islam video continues to draw scrutiny and criticism.

Fox News reports that administration officials knew within 24 hours that the Benghazi attack was terrorism.

On Thursday Defense Secretary Leon Panetta became the latest senior administration official to affirm that the assault in Benghazi, which killed U.S. ambassador Chris Stevens and three other Americans, was "a terrorist attack."

"The reason I think it pretty clearly was a terrorist attack is because a group of terrorists obviously conducted that attack on the consulate and against our individuals," he told a Pentagon briefing.

"What terrorists were involved I think still remains to be determined by the investigation. But it clearly was a group of terrorists who conducted that attack against that facility."

Asked when he had reached this conclusion, Panetta ducked: "As we determined the details of what took place there, and how that attack took place, that it became clear that there were terrorists who had planned that attack, and that's when I came to that conclusion."

"It took a while to really get some of the feedback from what exactly happened at that location," he added.

Rice, meanwhile, looks increasingly isolated on the matter, and Republican senators are seeking answers.

"In the aftermath of the September 11 terrorist attack in Benghazi that resulted in the death of four Americans, including Ambassador Chris Stevens, you made several troubling statements that are inconsistent with the facts and require explanation," Republican Sens. John McCain (Ariz.), Lindsey Graham (S.C.), Kelly Ayotte (N.H.) and Ron Johnson (Wisc.) said in a letter to Rice on Wednesday.

"We look forward to a timely response that explains how the U.S. ambassador to the United Nations could characterize an attack on a U.S. consulate so inaccurately five days after a terrorist attack that killed four Americans."

Two other Republican senators are also pressing the administration for answers - not specifically about Rice's comments but about security threats in Libya in the run-up to the Sept. 11 attack.

In a letter to Clinton this week, Sens. Johnny Isakson (R-Ga.) and Bob Corker (R-Tenn.) asked her to "transmit to the Senate Foreign Relations Committee all communications between the U.S. Mission to Libya and the State Department relevant to the security situation in Benghazi in the period leading up to the attacks, including, but not limited to, cables sent from Ambassador Stevens."

White House press secretary Jay Carney on Thursday accused Republicans of politicizing the issue.

"Every step of the way, the information that we have provided to you and the general public about the attack in Benghazi has been based on the best intelligence we've had and the assessments of our intelligence community," he told reporters en route to an Obama campaign rally in Virginia Beach.

"We have said all along that there's an ongoing investigation and that as more facts come out, we will follow those facts wherever they lead and apprise you of our assessments as those facts come to light," he added.

"What has also been the case is that from the very first hours after the attacks and the unrest in Cairo, there has been an attempt, unfortunately, by Republicans, beginning with Governor Romney, to try to turn this event into a partisan issue, to try to score political points out of a terrorist attack that cost the lives of four Americans, including our ambassador to Libya - and that's unfortunate."

Carney reiterated that the president views the Benghazi assault as a terrorist attack, although the president himself has not used the word terrorism.

Timeline of administration statements two weeks ago:

Sept. 14 (late morning): White House press secretary Jay Carney tells a daily press briefing:  "Let's be clear, these protests were in reaction to a video that had spread to the region." Asked whether that applied to Benghazi, he replied, "We certainly don't know. We don't know otherwise.  We have no information to suggest that it was a preplanned attack."

Carney went on during that briefing to say repeatedly that the violence was a response to the video:

--"It is in response to a video, a film that we have judged to be reprehensible and disgusting."

--"This is in response to a video that is offensive to Muslims."

--"The unrest we've seen around the region has been in reaction to a video that Muslims, many Muslims find offensive."

--"Again, what we have seen is unrest around the region ...