Sen. John McCain, R-AZ, Sen. Lindsey Graham, R-SC, and Sen. Kelly Ayotte, R-NH, hold a news conference on the Benghazi terrorist attack at the U.S. Capitol, Nov. 14, 2012, in Washington. (Image credit: Win McNamee/Getty Images)

Update at 6:29 p.m. ET:

A spokesman for Sen. McCain points out that he spent hours at the Intelligence Committee briefing Thursday afternoon, and will be at the committee's hearing with General Petraeus Friday morning.

Update at 2:05 p.m. ET:

White House Press Secretary Jay Carney called the scheduling mistake that sent Sen. John McCain to a press conference on Benghazi instead of a classified briefing "extraordinary."

"It is extraordinary that Senator McCain continues to go on television demanding answers on Benghazi, but refused to go to a classified briefing where he could get those answers," said Carney in a statement to ABC News.

Original post 12:05 p.m. ET:

Senator John McCain is demanding answers on the Benghazi attack, but his office tells ABC News he missed a classified briefing on the subject because of a "scheduling error."

The classified briefing was held on Wednesday before the Senate Homeland Security and Government Affairs Committee - of which Senator McCain is a member - and lasted three hours. It featured testimony by officials from the State Department, the Pentagon, the CIA and the National Counterterrorism Center.

During part of the briefing, McCain was holding a press conference demanding answers about the administration's handling of the attack on the U.S. Consulate in Benghazi, Libya on Step. 11 that killed four Americans, including US Ambassador to Libya Christopher Stevens. McCain called on Congress to launch Watergate-style hearings to get to the bottom of what happened.

"More than two months after the Benghazi attack, there are still many unanswered questions," McCain told reporters in the Capitol Hill press conference. "While we await the findings and recommendations of the administration's internal review of the Benghazi attack, it's essential for the Congress to conduct its own independent assessment."

At precisely the same time McCain was holding that press conference, the briefing for the Homeland Security Committee was happening in another part of the Capitol building.

Why did he missing the briefing on a subject he has been so adamant in demanding answers on? McCain's office says his absence was unintentional - an oversight.

"Senator McCain was absent from the hearing due to a scheduling error," McCain spokesman Brian Rogers told ABC News.

Even if he had attended, McCain was unlikely to be satisfied with what he heard.

After the briefing was over, the top Republican on the Homeland Security Committee, Senator Susan Collins, was asked if she was satisfied with the hearing.

"I really wasn't," Collins answered. "There are many, many unanswered questions. I feel that we've only scratched the surface through the briefings that we had today."

There are currently at least four Senate Committees looking into the Benghazi attack. For her part, Sen. Collins does not agree with McCain's call to combine those into one special - or "select" - committee like the one that investigated Watergate.

" I do not see the benefit of, nor the need for a select committee," Collins said. "Our committee, our Homeland Security Committee has government-wide jurisdiction and a history of producing comprehensive bipartisan reports on everything from the Ft. Hood terrorist attack to Hurricane Katrina, so I don't see the need for creating a brand new select committee to take a look at this."

The White House had no immediate comment on McCain's absence from the hearing, but one former White House official was quick to jump on it.

"It is nothing short of appalling that Senator McCain would use his time and influence to play politics instead of getting answers to the questions he claims to have," said Bill Burton of the pro-Obama group Priorities USA.

-Jonathan Karl and Sunlen Miller