The room is soundproof, the Capitol Police are standing guard, and the members of the House and Senate joint investigative panel have convened at the fourth story converted attic space in the Capitol that passes muster as a secure room for Tuesday's closed hearing on intelligence failures.

The meetings, which will try to connect the pieces of a puzzle that could have prevented the terror attacks, started with a boost of support from President Bush, who said that he supports a congressional inquiry as long as it doesn't detract from the work intelligence agents must do in order to prevent future terror attacks.

"What I am concerned about is tying up valuable assets and time and possibly jeopardizing sources of intelligence. And that's why it is very important that the Congress do investigate, but they do so in a way that doesn't jeopardize our intelligence-gathering capacity. That's why they have intelligence committees on Capitol Hill, and that's the appropriate form, as far as I'm concerned, for these investigations," he said while touring the National Security Agency Tuesday.

Bush said all kinds of speculation has flown over what U.S. intelligence agencies knew and when they knew it, but Bush said he doubted that if anyone had enough information to stop the attacks, they wouldn't have acted on it.

Tuesday's hearing will look into old information and new, like Monday night's revelation that the FBI learned from the CIA as early as January 2000 that hijackers were planning a Sept. 11-style attack.

Of course, the information comes from a CIA official. The tit-for-tat has turned from dribbles to streams of finger-pointing accusations over which organization dropped the security ball.

Whatever the case, members of the intelligence committee want to know what the agencies are going to do about it in the future.

"We need to be aggressive and rigorous in this inquiry, asking the right questions like who knew what? And if they didn't know it, why? And what did they do with the information they had?" Sen. Barbara Mikulski, D-Md., one of 20 senators on the intelligence panel, said as she headed into the hearing.

FBI Director Robert Mueller has already laid out the plans for an overhaul of the structure, culture and mission of the FBI. It's a first step, say lawmakers, who appear less interested in placing blame than in preventing future acts of terror.

"The test will be in the performance in the future, not just the arranging of the new rules," said Sen. Richard Shelby, R-Ala., ranking member of the Senate intelligence panel.

Nonetheless, several gory details of infighting among agents and cross-bureau miscommunications were being mulled in Tuesday's closed hearing. Lawmakers were focusing on a report that says U.S. agents had infiltrated the Al Qaeda terrorist network and overheard the terrorists plotting a major attack weeks before Sept. 11. Among the CIA papers turned over to the intelligence committees are memos indicating Al Qaeda intended to attack unspecified American interests.

Al Qaeda communications were also intercepted. Listening stations heard operatives using cryptic phrases like, "watch the news," and "tomorrow will be a great day for us." The question is whether this information, much of it vague and non-specific, could have been connected together and used to prevent the attacks.

Officials were also discussing what information the CIA knew and shared about Khalid Almihdhar, one of the alleged hijackers of the plane that crashed into the Pentagon.

One anonymous CIA official said the CIA had identified Almihdhar by name as one of the attendees at the then-upcoming meeting of suspected Al Qaeda members in Kuala Lumpur, Malaysia, which took place in mid-January 2000. They also knew his passport number, birthdate and that he had a multiple-entry visa allowing him into the United States.

Almihdhar was at the meeting with another man, Nawaf Alhazmi, who was also on the Pentagon-bound flight. In Malaysia, they met with a man later discovered to be the alleged mastermind of the USS Cole bombing, which took place 10 months later. But it was reported CIA officials did not know his name until two weeks before the Sept. 11 attack.

Some officials had accused the CIA of mishandling the Malaysia meeting and keeping the information to itself, preventing the FBI from tracking the terrorists once they entered the United States. The FBI declined comment on a Newsweek story that had suggested as much, saying Mueller is not interested in casting blame.

But the CIA official read from an FBI counterterrorism agent's Jan. 6, 2000, e-mail to CIA officials, in which the agent asks whom to ask for more information on Almihdhar. The CIA's response contains the name of two FBI officials who had been briefed on the suspected terrorist. A second CIA internal communication, sent Jan. 5, also says the FBI had been informed of the Malaysia meeting. The FBI was not aware, however, of Almihdhar's travels to the United States, a government official said.

Neither the CIA nor the FBI was able to learn of Almihdhar's plans for Sept. 11, and CIA officials acknowledged his case could have been handled better.

It is this months-long back and forth that will be discussed in the intelligence committee's closed-door hearings on Wednesday and Thursday.

Members will delve into the details surrounding Egyptian President Hosni Mubarak's charge that his country had informed U.S. officials about an attack being prepared by Al Qaeda. Mubarak, who is meeting with President Bush this weekend, told The New York Times that the warning was about a week before Sept. 11. A U.S. intelligence official denied the CIA received such information.

The committee will also look into the government's handling of Zacarias Moussaoui, an Islamic extremist who was arrested in Minneapolis in August and later accused of conspiring with the Sept. 11 hijackers.

Coleen Rowley, the Minneapolis FBI agent who said bureau headquarters ignored her office's pleas to investigate Moussaoui further, is expected to testify before the Senate Judiciary Committee on Thursday.

The CIA had learned of Moussaoui as early as the spring of 2001 but under another name — the informant knew him only by an alias. French intelligence told the CIA about Moussaoui in mid-August, but the agency didn't match the two names until after the attacks.

In addition, the FBI has been criticized for failing to link Moussaoui to the warnings of a Phoenix field agent that Middle Eastern men were training at American flight schools.

"We have a whole roomful of Coleen Rowley-type documents right now. I won't say they're exactly the same or quite as sensational, or speak to a leader or an agency's motives, but we have got all kinds of materials that we are reviewing with professional staff. That's why we're having the investigation and I believe most of that is going to be of interest to the American public," said Rep. Porter Goss, R-Fla., chairman of the House Intelligence Committee.

Fox News' James Rosen, Catherine Herridge and Collins Spencer and the Associated Press contributed to this report.