Amid congressional concern about NASA's objectivity in the wake of the Columbia disaster Saturday, the quasi-independent Accident Investigation Board, beefed up with non-NASA staff and board members, will assume the mantle of sole authority in determining what caused the crash that claimed the lives of seven astronauts.

AIB board chairman Harold W. Gehman, Jr., a retired Navy admiral, flew to Houston Wednesday to set up a base of operations at the Johnson Space Center. NASA Administrator Sean O'Keefe said today language in the charter that set up the AIB will be modified immediately to guarantee the panel's complete independence from NASA.

"We want the facts and the evidence to speak to the conclusions, ultimately inform the conclusions in terms of what caused this horrific event," O'Keefe said. "And in doing so, we do not want to rule out any theory, any approach, any possible set of factors that could be, when combined, lead to some other judgment. That is a determination that the Columbia Accident Investigation Board will render.

"As a consequence there ... we have been advised by the board that certain clarifications of the charter to assure the board's independence be modified. Those modifications we have immediately agreed to. ... This is to absolutely guarantee that we've eliminated any ambiguity as to the independence of this board."

In the days following the accident, NASA own internal investigation has proceeded around the clock. Daily briefings by shuttle program manager Ronald Dittemore served to inform the press and public about the progress of the probe. In stark contrast to NASA's policy after the 1986 Challenger disaster, Dittemore provided details about telemetry from the shuttle and how the spacecraft behaved in its final minutes.

And therein was a potential problem.

On Sunday, Dittemore told reporters telemetry indicated elevated temperatures and sensor failures in the left wing of the orbiter followed by the development of unusual aerodynamic drag along the left side of the fuselage. The spacecraft eventually veered out of control and broke up, presumably due to aerodynamic stress. At issue is the root cause of the failure, i.e., what caused the observed temperature increases and the aerodynamic drag.

Launch video shows debris from the external tank striking the underside of the shuttle's left wing 81 seconds after liftoff Jan. 16. On Monday, Dittemore told reporters one team of engineers was assuming the debris was the root cause. This is standard practice in a fault-tree investigation.

"We have a team of engineers and managers and technicians that are ... making the assumption from the start that the external tank was the root cause of the problem that lost Columbia," he said at the time. "That's our starting point when we look at the tank. And based on that assumption, what is the fault tree that would substantiate that particular assumption? And so we're attacking that. And that's a fairly drastic assumption and it's sobering."

Other teams of engineers were considering other branches of the fault tree, assuming different types of failures in a bid to rule out false trails and to zero in on the actual problem. Again, all standard procedure.

But many in the media reported that the debris impact on the tank was the prime suspect in the shuttle disaster. That is not what Dittemore meant, this writer believes, but that was the impression.

On Wednesday, after a memorial service Tuesday to honor Columbia's crew, Dittemore responded to the media accounts by saying he personally did not believe the tank foam could have caused enough damage to result in a loss of the vehicle. And he said, rather emphatically, that he did not believe ice on the tank was a contributing factor. The media then reported NASA had "backed off" the foam theory.

The back-and-forth coverage of the investigation may have ruffled feathers in Washington. In any case, it likely added to concern among lawmakers demanding a fully independent panel of investigators in the mold of the Rogers Commission that investigated the Challenger disaster (see separate story).

"We would respectfully ask that the charter of the AIB be re-drafted to reflect a broad mandate encompassing contributory causes, management issues, and pressures on the system," Rep. Ralph M. Hall (D-TX), Rep. Bart Gordon (D-TN) and others wrote President Bush on Thursday. "In addition, we would recommend that, to ensure its complete independence, the AIB should report directly to you and to the Congress and that the support staff for the Board should come from agencies other than NASA."

O'Keefe said late this afternoon that Gehman and his panel would have complete independence. Apparently reflecting outside concern that a possible failure mode - debris from the tank - had been eliminated from consideration, O'Keefe said "we really want to be sure there is no ambiguity whatsoever and that we are not eliminating any set of possibilities of what could have contributed to this accident."

"The intention here very clearly is they (the AIB) will reach conclusions and the conclusions will come from them and only them," the administrator said. "We will not have competing positions on this. ... The board findings are what we will be guided by."

O'Keefe closed his remarks by saying the first congressional hearing on the shuttle disaster is now scheduled for next Wednesday in Washington.

Dittemore then held his afternoon briefing as usual and said from a personal standpoint, "I'm very pleased to have Adm. Gehman here with his team and we offer our full support. Anything that Adm. Gehman needs, we will provide to him. And we pledge to him that we will continue to support him as we have done in the past. As we have prepared investigation plans and procedures, we will transition our thoughts and our plans over to the board and we'll follow his leadership."

Dittemore said so far, more than 1,000 pieces of shuttle debris have been recovered. But no confirmed shuttle debris has so far been identified west of Forth Worth, Texas. He said engineers are continuing to study telemetry from the shuttle and are continuing to develop the fault tree of possible causes.

"Let me emphasize again, we have not ruled out any possible cause," Dittemore said today. "And even though we scratch our heads from time to time and wonder if we're going down a right path, it's important to understand that the first step in any investigation is to develop a fault tree.

"And in the fault tree, you examine and identify every possible cause no matter how remote or no matter what you think about the possibility of that cause. And once you develop that tree, then you establish a process where you systematically close out each branch of the tree. So in the end, you have left no stone (unturned). You have looked at each branch, each possible block, each possible cause and satisfied yourself that you have done a thorough and complete job, independent of what you might think of the probabilities or possibilities.

"As I talked to you yesterday, I mentioned to you that we believe in some instances that it's hard for us to understand how a piece of foam that has flown off the tank could have been the root cause. But that is not stopping us from continuing to investigate that particular event as being a potential root cause. We are planning testing of foam impact on tiles. We're performing analyses. That's just an example, that even though we scratch our heads and don't quite understand how this could be a contributor, it still exists in the fault tree and we're going to pursue that branch of the fault tree until we close it as a possibility.

"We're still looking for that elusive missing link," he said. "And we're hopeful that as we examine our fault trees across the program and perform our testing and look at and examine the debris we have gathered that we will find the missing link. That may take some time to do. You can imagine if you look at the fault tree, it's going to have thousands of blocks to pursue, many different branches to follow. So systematically, the large team that we have gathered will go through each one of those branches and this activity again will be led by the accident investigation board and we will support that activity to the best of our abilities.

"Again, let me say it is with some relief that I welcome Adm. Gehman here," Dittemore said. "We need their expertise, we need their independent look at what we have been doing and we will work closely with him and his board."

Dittemore did not provide any new details today, but he said he hopes to present graphics Friday that will overlay Columbia's ground track across the southwestern United States with time-coded telemetry readings showing when data from the shuttle indicated problems.

He was asked today about amateur video shot in Reno and north of Flagstaff that show what appears to be debris breaking off the orbiter well before the shuttle's breakup over Texas.

"I have seen some of the video," Dittemore said. "It's interesting, but we haven't yet determined what's happening, obviously. As far as any possible pieces of the orbiter or tile falling off, it doesn't appear to show up on any of our data as affecting our flight control until later on in the time frame that we have discussed.

"In that (early) time frame as we look at the data, unless you had some pictures indicating an event was happening, you wouldn't be able to tell by looking at the system's performance or even the flight control handling qualities, it looks normal to us. And that's part of the mystery. If there is something that's shedding from the orbiter, we'll have to determine that to be the case because it doesn't show up in the data."

Asked about the morale of the NASA team in the wake of the disaster, Dittemore said "certainly, there has been trauma in our system. ... But I think the healing process has begun."

"I'm seeing a very professional manner in all of our teams, a resolve to get to the bottom of this, a determination to find a root cause and I know that to a person we will not rest until we have solved this problem, until we have put all the pieces together, until we find that missing link.

"And we will support the leadership of our investigation board to make sure this happen. These five days have been exhausting to the team. They have been difficult emotionally and physically. But we're going to carry on and we're going to continue our determination to find the root cause and do so as quickly and reasonably as we can."