NASA has all but ruled out that the Columbiadisaster was caused by foam debris that broke off the fuel tank at lift-off.
NASA shuttle programme manager Mr Ron Dittemore said the piece of foam that broke off the external fuel tank shortly after lift-off was about two-and-a-half pounds and was moving about 750 feet per second, neither heavy enough nor fast enough to damage the shuttle's heat-resistant tiles.
"We're focusing our attention on what we didn't see. We believe there was something else," Mr Dittemore told reporters. "There's got to be another reason".
Mr Dittemore said the foam insulation, which coats the large, orange tank beneath the orbiter at launch, was impervious to moisture. Reports have suggested the foam may have become waterlogged and that the water turned to ice.
"There is a missing link," Mr Dittemore said. "Right now it just does not make sense to us that a piece of debris would have been the root cause for the loss of Columbia and its crew".
Although NASA is considering such possibilities as a meteoroid impact or collision with floating space junk while Columbiawas in orbit, the agency still focused on some launch-related event as a likely cause of the event.
Near the Kennedy Space Centre, workers have scoured the beaches Columbiaflew over to see if anything fell off the shuttle and landed there. They are also studying ocean currents and testing whether various parts float.
The search for debris from Columbia's crash widened into California and Arizona, where parts may have fallen away before NASA lost contact with the shuttle over Texas.
US attorneys said two people had been indicted for stealing debris.