WASHINGTON Although the first shipment of debris from the shuttle Columbia has arrived at Barksdale Air Force Base, La., NASA officials said their multi-state search still has not produced any of the high priority items such as Columbias left wing or the heat protection tiles on that wing. It is those missing high priority items that the agency hopes will hold the clues to Columbias mysterious disintegration over Texas Feb. 1.

In a press conference at Johnson Space Center in Houston Feb. 5, Ron Dittemore, NASAs shuttle program manager, acknowledged that the search is spreading west in response to eyewitness reports that Columbia may have started to break up by the time it was over California.

"If we found debris in California, New Mexico or Arizona it would certainly be significant, and the pieces of debris would certainly be significant," Dittemore said.

He cautioned, however, that as the search moves west, the difficulty of finding key debris will be much greater because the search area will dwarf the debris field in East Texas where most pieces have been found so far. Dittemore said it would take NASA scientists some time to calculate the size of the debris footpath, which would be spread over a wide area because of the high altitude of Columbia as it traveled over the western United States and its high rate of speed at that time.

One of the reasons investigators are so eager to find more key pieces of debris is that they are more convinced than ever that the root cause of the accident is not the piece of insulation from Columbias external fuel tank that struck the orbiters left wing during its launch from Cape Canaveral.

Dittemore said further analysis of that incident indicated that any damage from the insulation would have led to the loss of the orbiter.

"As you focus on debris, were focusing on what you didnt see," Dittemore said. "Right now it just does not make any sense to us that a piece of debris would be the root cause of the loss of Columbia and its crew."

NASA analysis found the foam would not have caused any damage, and launch video showed the foam disintegrating once it hit the orbiter, Dittemore said.

NASA has not narrowed down its search for the root cause of the problem that led to the destruction of the orbiter and the agency continues to investigate multiple possibilities, Dittemore said. "It has got to be something else that we dont know about yet," Dittemore said.

More debris is expected to arrive at Barksdale before this weekend, Dittemore said.