A key Republican on the Senate Intelligence Committee has signaled his support for an independent commission to study intelligence and law enforcement failures that led to the Sept. 11 terror attacks.

Sen. Richard Shelby of Alabama, the panel's ranking GOP member, said Wednesday that his decision to support the creation of an independent commisssion is based on concerns that the FBI, the CIA and other agencies are stonewalling the committee's inquiry into what they knew and when they knew it, and are not turning over complete information.

The congressional investigation is a "prelude to further inquiry," Shelby said, adding that an additional study is still needed, due to a lack of cooperation and candor from various agencies.  Several commitee Democrats have come out in accordance..

"Our inability to detect and prevent the Sept. 11th attacks was an intelligence failure of unprecedented magnitude," said Shelby. "Some people who couldn't seem to utter the words intelligence failure are now convinced of it."

The Bush administration and GOP leaders oppose such a study, saying it could lead to leaks. The White House thinks the joint intelligence committee can conduct a definitive and singular review.

Shelby's announcement comes as Eleanor Hill, staff director for the House and Senate intelligence inquiry, said that Congress had been briefed two months before the Sept. 11 attack, and warned that Usama bin Laden would launch a spectacular terrorist attack against U.S. or Israeli interests.

The briefing, for senior government officials, was part of "a modest, but relatively steady stream of intelligence information indicating the possibility of terrorist attacks inside the United States," said Hill's 30-page statement.

The committee's interim report chronicles a series of missteps in U.S. intelligence agencies, and lists specific clues that an attack on the United States was coming.

According to the committee's inquiry, between June of 1998 and Spring of 2001, there were at least 17 specific intelligence indicators that Usama bin Laden planned to attack the U.S. homeland.

Between March 2001 and September 2001, there were 11 specific intelligence indicators of an imminent attack in the United States

Between December 1994 and September 2001, there were 12 specific intelligence indicators that Al Qaeda was planning to use aircraft to attack U.S. targets, particularly in New York City and Washington, D.C.

Hill qualified that the knowledge was only partly useful since the credibility of the sources was sometimes questionable and specific details were unavailable in many cases.

"They generally did not contain specific information as to where, when and how a terrorist attack might occur and generally are not corroborated by further information," her statement said.

Hill's statement were presented to committee members Wednesday at the inquiry's first public hearings. Lawmakers have been meeting behind closed doors since June, looking into intelligence failures leading up to the attacks and how they can be corrected.

"These public hearings are part of our search for the truth -- not to point fingers or pin blame, but with the goal of identifying and correcting whatever systemic problems might have prevented our government from detecting and disrupting Al Qaeda's plot," said Sen. Bob Graham, chairman of the Senate Intelligence Committee, as the hearing opened.

A senior CIA official told Fox News that the committee report is "largely accurate," but offered an interpretation that differed from the one congressional sources proposed.

For one thing, the official said, the report makes clear that the intelligence community generated a lot of information on the terrorists and a lot of warnings that were shared with the FBI, the White House and the State Department, among other agencies.

In addition, he reinforced Hill's statement that most of the warnings "did not contain any specific indication of where, when or what" the terrorists might be planning and specific information came from sources of "varying reliability."

Most indicators were "strategic, not tactical," alerts, the official said, meaning they were about a general threat, not about specific, imminent acts.

The official also noted that the reports in the weeks and months before the attacks - including some 33 National Security Agency intercepts between May and July 2001 - indicated "possible imminent terrorist attack" but none could direct intelligence agents to specific dates and places.

"Taken in their entirety," the official concluded, "it is unclear they referred to the Sept. 11 attacks."

The CIA views the report as an indication of just how aggressive the intelligence community was in going after Usama bin Laden, especially in the last three or four years.

"It's a good report that says the intelligence community engaged in numerous attempts to collect intelligence on Usama bin Laden's network and to disrupt his operations," said the official.

But that wasn't good enough for leaders of two groups of victims' relatives, Stephen Push and Kristin Breitweiser, who both lost spouses in the World Trade Center and Pentagon attacks.

Breitweiser, whose husband Ron died at the World Trade Center, told lawmakers that if the public had been aware of possible terrorist attacks, airport security could have been bolstered and passengers may have thought twice before boarding airplanes on the day of the attacks.

"How many victims may have taken notice of these Middle Eastern men that were boarding their plane?" said Breitweiser, of Middletown, N.J. She is co-founder of September 11th Advocates.

Fox News' Jim Angle and Carl Cameron and the Associated Press contributed to this report.