The U.S. space agency, NASA, has released documents showing that the doomed shuttle Columbia was struck by three pieces of stiff insulating foam at launch, not one as noted previously. Investigators are probing the possibility that the foam impact may have led to Columbia's demise upon landing.

A report from a shuttle subcontractor, the Boeing Company, says Columbia's external fuel tank shed three pieces of briefcase-sized foam during launch. The report is dated one week before the shuttle disintegrated on February 1.

Since the disaster, NASA has spoken of only one piece of foam hitting the orbiter's left wing.

Shuttle officials have said that single piece was unlikely to have had an impact on the wing's heat-protecting ceramic tiles, even at an estimated impact speed of 200 meters per second. Whether they knew about the second and third impacts is unknown.

The independent board investigating the disaster says something apparently breached Columbia's left wing, allowing superheated gases to penetrate before the shuttle broke up upon re-entry. It is investigating the foam strike as one possible cause of the failure.