The chairman of the independent Columbia Accident Investigation Board - AIB - said today the root cause of the Feb. 1 shuttle disaster may never be known, but he vowed to leave no stone unturned in a herculean effort to nail down exactly what triggered NASA's second shuttle disaster.

Speaking at the board's first news conference, retired Navy Adm. Harold Gehman said his panel, including some of the nation's leading accident investigation experts, will conduct a fully independent probe beyond any possible influence from NASA.

It is an "extraordinarily rich and deep board in accident investigations and aircraft matters," Gehman said of his colleagues. "Among them they have been either the chief investigator or an investigator on essentially every major aircraft mishap in the United States probably for the last couple of years. Each of them is qualified to be the chairman of this board.

"They are probably the most senior and the most talented aviation mishap individuals in the United States of America. They got that way because of years and years of hard work, independence and integrity and I can assure you nobody is going to put that reputation on the line and allow themselves to be influenced by any outside influences."

In the end, he said, "there is only one investigation going on. And it is our investigation."

He said the AIB will continue NASA's policy of openness, saying raw data, debris reports, imagery and other bits of evidence and data pertinent to the investigation will be released as it is collected and reviewed. At the same time, data and other impounded items will be released to the public as soon as the board determines nothing will be lost to the investigation.

In one of the board's first acts along those lines, the mission control audio tape recorded the morning of Columbia's re-entry was released today, providing a compelling glimpse into how the flight control team struggled to make sense of the sensor failures and unexpected readings that preceded the shuttle's breakup.

Gehman also said the AIB is establishing its own data and analysis review committee to double check any technical conclusions reached by NASA to independently verify their accuracy.

"It's not that we don't believe the NASA data, it's just that in order that the report be truly independent and that the conclusions be based on good solid analytical work, both inside NASA and outside NASA, this team will advise us on when we come across one of those junctures in the road," Gehman said. "They will also write the statement of work, they will also liaison with whatever laboratory or university that's doing the independent analysis."

The board's charter gives the team 60 days to reach a conclusion. But Gehman said that is more of a guideline than a firm deadline.

"We are driven by two imperatives," he said. "The first imperative is to get it right for a number of reasons, including the safety of follow-on shuttle crews, not to mention the continuation of the program. That imperative drives us to take our time and be very meticulous.

"We have another imperative, which is the welfare of the three astronauts who are on orbit right now who depend on the shuttle system, at least for their transportation if not for their supplies. In that sense, we have an imperative to move along as rapidly as we can."

Gehman said that as of today, no confirmed shuttle debris has been found west of Fort Worth. But numerous witnesses reported seeing what looked like debris falling away from Columbia as it streaked across California and Nevada on its way toward Texas.

"We have reason to believe, we don't have proof, but we have reason to believe we should keep looking west of Fort Worth," Gehman said. "The difficulty is as we reconstruct this accident in greater detail, the more data, the more pictures we get, the more videos we get, the more we examine the engineering data, the more we realize when this incident started. If there are credible possibilities west of Fort Worth, we're going to energetically pursue them because it's possible there's something out there."

But, he added, "there's a lot of empty space west of Fort Worth."

"And what NASA has been attempting to do since day one is to be more predictive as to where we should look for the debris," Gehman said. "We're trying to find radar tapes from Nellis Air Force Base, we're looking every place to see whether or not any unusual radar returns (are available). We're calling in many outside NASA and we have outside people now to look at the predictive analysis of where the debris might have landed."

So far, Gehman said, more than 1,600 pieces of shuttle debris have been identified. The first truckload of debris is now en route to the Kennedy Space Center where technicians will partially reconstruct the spacecraft in a large hangar near the shuttle runway. Gehman said part of the ship's left wing has been identified, but reports yesterday that one of Columbia's five general purpose computers had been located were incorrect. Instead, searchers found the electronics box used by the shuttle's KU-band television antenna system.

While vowing to conduct an open, independent investigation, Gehman said the goal was simply to find out what went wrong and that his team has no interest in placing blame.

"We aren't out to find any guilty people or any negligence or culpability," he said. "But if you have a secretary in a potted plant outside your office, then you're fair game."

Asked if he is confident about finding a single root cause of the Columbia disaster, Gehman deferred to two of his board members.

Maj. Gen. Kenneth W. Hess, Air Force chief of safety and commander of the Air Force Safety Center at Kirtland Air Force Base, N.M.,. said in many cases accident investigators can only point out "what we most likely think may have happened. But you can never prove it with all certainty."

"And so we'll likely get ourselves into probable causes in any number of areas here in this particular mishap," he said. "Our charter is to try to find the cause and to the extent we can drill down and ask enough questions of why, we're going to attempt to do that. But it is a probable outcome that we may not find the exact cause of this mishap. But we will have to let the information speak for itself in the course of the investigation."

Rear Adm. Stephen A. Turcotte, commander of the Naval Safety Center in Norfolk, Va., said "if we do get to the point where we have not exactly found the mishap, we will have covered everything else, we will have looked at those positives, was it this, was it that ... so we'll have narrowed the focus.

"Looking at the complexity of this, it is huge," he said. "It is one of the biggest debris fields I think any of us have ever seen. The complexity of the science and technology that goes into it, it is huge. We're going to look at everything and we're going to narrow it down to the most probable cause, if not the cause."

The AIB members spent the morning flying re-entries in NASA's shuttle simulators at the Johnson Space Center. They are flying to the Kennedy Space Center Wednesday for two days of familiarization and tours before heading on to the Marshall Space Flight Center in Huntsville, Ala. They plan to return to Houston next week.

Meanwhile, a joint congressional hearing is planned Wednesday beginning at 9:30 a.m. in the Russell Caucus Room of the Russell Senate Office Building. NASA Administrator Sean O'Keefe and William Readdy, associate administrator of space flight, are scheduled to testify.

The joint hearing, chaired by Sen. John McCain and Rep. Sherwood Boehlert, will be carried live on NASA television.