The Columbia Accident Investigation Board, reporting solid progress in its ongoing probe of the Columbia disaster, has not yet pinned down the location of catastrophic breach that allowed superheated air to enter the doomed ship's left wing during re-entry Feb. 1. But intriguing deposits of aluminum-and-steel slag behind panels making up the leading edge of Columbia's left wing near where it joins the fuselage suggest the breach may have occurred toward the front of the wing, close to the shuttle's fuselage, allowing hot plasma to work its way through the wing's interior with deadly results.

But investigators have not yet ruled out a breach in the wheel well itself, despite sensor data that fails to show the sort of blowtorch-like heating one might expect from a direct plasma intrusion. In the absence, however, of any wreckage from the left landing gear door - and because of unusual heat damage seen in pieces of the forward inboard landing gear door frame - investigators say a wheel well breach remains a possibility, albeit one that is difficult to explain at present.

"Our job is just beginning," said board member Roger Tetrault. "What we will be doing is trying to follow the heat. And I'm going to say that again. What we have to do is follow the heat. We will be doing this in order to back into the location of the original breach in the wing. We're going to be using all the tools at our command, including aerodynamic and thermodynamic computer models, we'll be using wind tunnel testing to do that and of course, we'll also use a sizeable amount of the telemetry that's available to us."

As of today, 22,563 pieces of shuttle debris have been recovered, including all six of Columbia's nose and main landing gear tires and 25 of the ship's 35 internal propellant and storage tanks. Of that total, 16,063 pieces of debris have been identified. Wreckage recovered to date totals 32,100 pounds, or about 13.7 percent of the shuttle's intact weight.

During a news conference today, Tetrault revealed that investigators have discovered a "thin black deposit" on heat shield tiles recovered from Columbia's left and right wings. The deposits show a high concentration of aluminum, which makes up the underlying structure of a space shuttle.

"If you look at the debris from the right side, you can see that there is significant damage to the right side from re-entry," he said. "We see the black deposits on the right side, not to the extent we see it on the left side, but it's on the right side as well, which means you had molten aluminum being sprayed or deposited onto those tiles on the right side where the event was not occurring. That's a very hot re-entry."

Both left-side main landing gear tires have now been recovered and both show signs of "extreme trauma," Tetrault said, in contrast to a tire from the right side that shows less extensive damage. The left-side tires may have overpressurized and exploded due to extreme heating as the shuttle broke apart 207,000 feet above Texas. But investigators do not believe tire detonation played a significant role in the disaster.

Top: Tire from right main gear. Bottom: Left main gear inboard tire.

"We've recovered both of the tires from the left wheel well and we've been able to identify their exact position inside the wheel well by the amount of patches that there were inside the tire," Tetrault said. "The tires on the left side obviously have a significant difference in appearance from the ones that are on the right side."

Based on the observed damage and failure patterns, "we believe it is possible - and I'll say that again, it is possible - that the tires on the left side blew very late in this event," he said. "This would have been a late event because we have data that indicates that the orbiter was flying under control until the last few minutes before breakup. The blowing of these tires would likely have been a very catastrophic event so it couldn't have occurred until late in the event."

Said board chairman Harold Gehman: "We have telemetry from the wheel well up until the time of loss of signal that indicates those tires were intact, they had the right air pressure and they had the right temperature. So whatever happened, happened after the loss of signal."

In addition to both left-side main landing gear tires, recovery teams have found an actuator used to move Columbia's left inboard elevon, or wing flap. The actuator has a hole in it, apparently the result of a burn through during or just after vehicle breakup, measuring two inches by four inches. Interestingly, traces of hydraulic fluid from the actuator show no signs of heat-related distress.

Data in the final two seconds of telemetry from Columbia - a final burst that followed a 25-second loss of data - showed Columbia was still on course with its fuselage intact and its major systems still functioning. But that two seconds of data contained no telemetry from any systems in the left wing and showed the shuttle's hydraulic reservoirs were empty. By that point, Gehman said after today's briefing, the left wing was either gone or so severely damaged the triply redundant hydraulic lines running into the wing had been severed.

That much had been surmised before. The most intriguing bit of news from today's briefing involved deposits of aluminum and steel slag on the inner surface of U-shaped reinforced carbon carbon - RCC - panels that made up the leading edge of the left wing.

"We have identified at least one piece from sixteen of the 22 leading edge systems," Tetrault said. "These are either pieces of the reinforced carbon carbon, the RCC, or the structural components that attach them to the wing spare and those are made of stainless steel. In some cases, we have pieces of both. We ran some tests this weekend on RCC panel No. 9, or at least a portion of it, and there was slag on the inside of that RCC panel which we tested and it shows deposits of aluminum and stainless steel."

Many observers have speculated the failure of an RCC panel, damage to one of the "T-seals" that hold them in place or problems with the tiles that mark the boundary between the leading edge and the rest of the wing provide the best explanation for where the breach occurred. In this scenario, hot gas entered at or near the leading edge and then worked its way back to the wheel well, burning through sensor wiring and raising temperatures in brake lines and other systems as seen in the downlinked telemetry.

But Tetrault said engineers have not yet explained "how does (molten material) blow forward and how do you get stainless steel and aluminum up onto the back edge, if you will, of an RCC when in fact that stainless steel is behind the area?"

Rear Admiral Stephen A. Turcotte, another board member who spoke today, said analysts are studying how convection from a plasma burn through could affect interior wing components, possibly causing the kind of back-flow seen in the RCC slag deposits. While the board has not reached any conclusions, it is interesting to note that "the closer you get to the chine, or panel No. 9 where it (nears the fuselage), you see more molten (deposits). As you get farther away down the left edge of the wing, you see less molten spray," Turcotte said.

When asked whether investigators favor a leading edge failure or a wheel well burn through as the most likely cause of Columbia's loss, Tetrault said "I think those are both equally alive. Everybody has their own theory, I'm sure each of you have your own theory, everybody on the board has their own theory. I'm going to be patient and not express my theory at this time."

But when asked about telemetry from the wheel well, which showed a more gradual increase in temperature than one might expect from a direct burn through, Tetrault said "I'm having difficulty with some of the off-nominal timings."

"One of the reasons I'm having trouble is it's very simple, very simple physics," he said. "There was a temperature A brake line (sensor), hydraulic fluid temperature A, that went up very early in the event, it was either the second or the third one that went off nominal. Temperature B, which sits about two inches away from it, did not rise until about a minute and a half later whereas temperature C, which is probably six feet away, and temperature D, which is four feet away, are all rising off nominal.

"It doesn't make a lot of physical sense to me," he said. "What we found as we looked at these temperatures is that it appears to be a straight line up and at some point, NASA has called it off nominal. And there may be some variability on where that call is on where it is off nominal. So what I'm trying to tell you is if you're trying to put together a timeline, I think you can be fairly certain when (a sensor) went off line. But when it says it's off nominal, I think you're going to have to take that with a little bit of a grain of salt."

Asked about the relationship between Columbia's attitude, or orientation, and telemetry indicating elevated temperatures, Tetrault said "it's interesting to note that all of them were going up off nominal but then they went up in a very, very sharp fashion as soon as it rolled into the left wing down attitude. I won't say anything more, but it's interesting to note that that occurred that way."

Columbia had completed a right bank, or roll reversal, and was banking to the left when the ship broke apart.

Adding yet another piece to the puzzle, Gehman said a closer analysis of the telemetry from Columbia shows the ship's flight computers began working to counteract atmospheric drag on the left wing earlier than investigators originally thought and that they applied quite a bit of muscle in the final seconds to keep the ship on course.

"Even at the time of the final two seconds (of telemetry), the vehicle's attitude and position were correct," he said. "We do believe that the vehicle was fighting forces more strongly, the fight was getting a little more vigorous at that point and we also believe that the beginning of this, of some of the control measures that the vehicle was taking to maintain its attitude, started earlier than we previously thought."

In the meantime, he said, the board's computer modeling is getting more accurate and sophisticated.

"We are still trying to do what we call a backfit," he said. "We're trying to find a scenario which fits the temperature readings. And we are inducing (virtual) holes and we are inducing heat flow into the vehicle in various places and we are more sophisticatedly modeling how the heat flows in through all the little openings and cubby holes and things like that. So we don't have anything to tell you that's on the conclusion side, all I can say is the analysis is getting more sophisticated and we're doing more of it."

At the beginning of today's briefing he said the board "remains completely determined and energized to finding the answer to this problem. We are still working seven days a week, our energy and our seriousness have not flagged, we still have confidence that we're going to find the cause, the direct cause, and determine the contributing causes. We're dedicated to that end and we have no slacking off and we're not getting discouraged just because we haven't found it so far."

Said Tetrault: "I think it would be fair to say we have more questions than answers right now. But we're getting smarter fast and I believe there's a very good chance that we will, in fact, be able to localize the breach that occurred in the left wing. And we certainly need to do this in order to determine the cause of the accident. Until we have determined that location of the breach, every postulated cause of the accident is really just a theory."

In other developments, Gehman said today NASA Administrator Sean O'Keefe has agreed to reassign unnamed senior shuttle managers currently active in the disaster investigation to avoid any possible conflicts of interest. O'Keefe initially balked at Gehman's request. While no names have been officially mentioned, several sources have said the board was concerned about the ongoing involvement of several high-ranking shuttle managers, including Linda Ham, a former flight director who served as chairman of NASA's mission management team for Columbia's flight.

"It has become apparent that some of the chief managers of the investigation which NASA and this board share are also members of these boards that we're going to be looking at," Gehman said today. "We are then put into the place of having the investigators investigate themselves."

"That's not exactly true because NASA is not investigating management issues. Only we are investigating management issues. But ... I can't possibly have key investigatory managers also be the people whose performance we're looking at in other areas."

He said he was satisfied with O'Keefe's response.