As SPACE.com reported earlier, the newly identified threat comes from the large explosive bolts, manufactured by SUMMA Technology, Inc., that keep the pair of solid rocket boosters securely attached to the space shuttle's orange external tank during roughly the first two minutes of flight.

Once the two boosters are exhausted, a pair of small explosions sever the attach bolts, permitting the spent boosters to fall away from the speeding orbiter. A bolt catcher built into the system is designed to prevent the severed bolt from posing a debris hazard to the orbiter.

Testing conducted at the direction of the board revealed that the bolt catcher is just strong enough to do its job and could fail during future flights. Weighing about 40 pounds, the severed bolt could do even more damage to the shuttle than the 1.6 pound piece of insulating foam investigators believe smacked the underside of Columbia's left wing shortly after liftoff.

Investigators do not know whether the solid rocket booster bolt catchers worked on Columbia's last flight. They do know, however, that ground radar at Kennedy Space Center appears to show that the solid rocket boosters shed a piece of debris 126 seconds into flight, a point in time that coincides with their separation from the rest of the vehicle.

"What we have here is the possibility that we have another source of debris," said board member U.S. Air Force Maj. Gen. John Barry.

However, neither telemetry nor other sensor data incidates that Columbia suffered another debris strike at that point in time, Barry said. For that reason, the board's suspicion remains focused on breakaway tank foam as the root cause of the Columbia accident.

"This has not changed our working scenario," Barry said "Our working scenario is still foam coming off the bipod and hitting the orbiter's wing."

Barry said he believes the board will probably identify the bolt issue as one that must be addressed before NASA returns the space shuttle fleet to flight operations.