Congress and the American people deserve to know exactly what the CIA and FBI knew before the Sept. 11 terrorist attacks for two fundamental reasons: the importance of accountability and the need for a serious culture change inside the intelligence community.

"Maybe" remains the most assertive answer to the question of whether the attacks might have been prevented, and nothing uncovered so far would have revealed plans to turn jetliners into fuel-laden missiles. So closed-door hearings that began yesterday on Capitol Hill ought not be about second-guessing or finger-pointing.

They should be about getting intelligence leadership to own up to clear failures to act on information they had months before the tragedies - admissions they adamantly avoided in the months following the attacks. And they should be about ensuring that the environment inside the CIA and FBI - which encourages secrecy over openness, slaps down those who speak up and caters to fiefdom-building rather than cooperation - is altered enough to prevent these kinds of intelligence failures from repeating themselves.

With each passing day, it becomes clearer that both agencies missed several opportunities to anticipate and perhaps disrupt the attacks.

The FBI had Zacarias Moussaoui (the alleged would-be 20th hijacker) in custody in Minnesota, and an agent there tried to alert her superiors to what she thought was a bigger plot involving him. But she was stifled by what she described as a "climate of fear" that has chilled aggressive law enforcement.

The CIA also knew months before Sept. 11 that two of the hijackers were in the country and that they were taking flying lessons and plotting with fellow members of the al-Qaida terrorist network. But that information wasn't shared with anyone outside the agency.

According to recent reports, the CIA's longstanding reluctance to share information with other agencies for fear of "compromising sources" led the agency to deny the FBI information about the two hijackers it learned were in the country.

Perhaps even worse, because the information that each agency had wasn't shared, no one was able to take a step back to see the larger picture. No one conducted the kind of analysis that intelligence experts now say could have linked all the hijackers and presented an opportunity to foil their plans.

Congress is now conducting hearings to figure out what went wrong at the CIA, and the FBI has announced a massive restructuring to focus more closely on anti-terrorist efforts.

But the kinds of failures that are being revealed each day suggest problems that go far beyond structure at the CIA and FBI. Each agency is so entrenched in its own world, so hobbled by its internal workings and short-sightedness that important information can become a guarded secret rather than a mutual tool for investigation.

If Sept. 11 was in any way a product of those deficiencies, then both agencies failed horribly to fulfill their one mission: the protection of national security from domestic and international threats. The goal of the investigations - in Congress and at the agencies themselves - must be to find out why those troubles exist and correct them before tragedy strikes again.