HOUSTON -- The Columbia accident investigation board raised the possibility Tuesday that an unusually strong wind shear a minute into the flight weakened the shuttle's left side. The board also suggested the age of the spacecraft may have contributed to the catastrophe.

Retired Navy Adm. Harold Gehman, the accident board's chairman, said he and others are trying to determine "whether or not NASA was alert enough, whether or not they were doing all the right things" to detect aging as a threat to the shuttle fleet.

"If you agree with the theory that complex systems fail in complex ways, it isn't a matter of just a bracket breaking. It's a matter of a whole series of unfortunate events," Gehman said at a news conference.

The leading theory is that damage to the left wing allowed hot gases to penetrate the shuttle and destroy it over Texas on Feb. 1, killing all seven astronauts.

A few pieces of insulating foam or other debris broke off the external fuel tank 81 seconds after liftoff and slammed into the leading edge of Columbia's left wing.

But the board also indicated the wing may have been made more vulnerable to debris damage because it was buffeted by unusual wind shear about 20 seconds earlier in the liftoff.

'Unhealthy' orbiter

The wind shear was within NASA's safety limits, but it was the strongest gust ever seen so close to the point where the shuttle is exposed to the maximum aerodynamic stress of liftoff, the board said. That point occurs around 80 seconds into a launch.

"It's possible that the foam striking a healthy orbiter would not have done enough damage to cause the loss," Gehman said. But he noted that Columbia may have been "unhealthy," because of the wind shear, aging or other factors.

"A normal event, which she could have survived at age 10, maybe she couldn't survive at age 21," he said.

Air Force Maj. Gen. John Barry, a board member, said the removal of the boosters may have introduced problems or weaknesses in the spot where the foam ultimately came off.

A visual inspection of the suspect foam area, called the bipod, was done and found nothing wrong, but he said more testing may have been required to find any cracks or other foam flaws.

"Our question really is ... is it adequate enough to just do a visual inspection," Barry said.

He said he and his colleagues are pursuing a "follow the foam" strategy in hopes of ascertaining whether that alone -- or in conjunction with other things -- damaged the protective heat panels on the leading edge of the left wing enough to allow the hot deadly gases of atmospheric re-entry to penetrate 16 days later.

"You have to address the issue of aging spacecraft in an R-and-D environment," Barry said, referring to research and development. "We've never been there before."

Barry said he is ordering an analysis of the heat-resistant carbon panels that line the leading edge of the wings, to determine the thoroughness of testing between flights. It is possible that voids could have developed under these panels, because of oxidation eating away at the carbon, he said.

"Think of termites," Gehman said.

One of the many questions, Barry said, is whether the panel inspections were good enough given the age of the shuttle fleet.

Columbia made the first shuttle flight in 1981.