CAPE CANAVERAL, Fla. -- Investigators trying to figure out what destroyed space shuttle Columbia immediately focused on the left wing and the possibility that its thermal tiles were damaged far more seriously than NASA realized by a piece of debris during liftoff.

Just a little over a minute into Columbia's launch Jan. 16, a chunk of insulating foam peeled away from the external fuel tank and smacked into the ship's left wing.

On Saturday, that same wing started exhibiting sensor failures and other problems 23 minutes before Columbia was scheduled to touch down. With just 16 minutes remaining before landing, the shuttle disintegrated over Texas.

Just the day before, on Friday, NASA's lead flight director, Leroy Cain, had declared the launch-day incident to be absolutely no safety threat. And an extensive engineering analysis had concluded that any damage to Columbia's thermal tiles would be minor.

''As we look at that now in hindsight, we can't discount that there might be a connection,'' shuttle manager Ron Dittemore said on Saturday, hours after the tragedy. ''But we have to caution that we can't rush to judgment, because a lot of things in this business that look like the smoking gun but turn out not to be close.''

The shuttle's more than 20,000 thermal tiles protect it from the extreme heat of re-entry into the atmosphere.

He said that the disaster could have also been caused by a structural failure of some sort.

As for other possibilities, however, NASA said that until the problems with the wing were noticed, everything else appeared to be working fine.

Dittemore said there was nothing that the astronauts could have done in orbit to fix damaged thermal tiles and nothing that flight controllers could have done to safely bring home a severely scarred shuttle, given the extreme temperatures of re-entry.

The shuttle broke apart while being exposed to the peak temperature of 3,000 degrees on the leading edge of the wings, while traveling at 12,500 mph, or 18 times the speed of sound.

Dittemore said that even if the astronauts had gone out on an emergency spacewalk, there was no way a spacewalker could have safely checked under the wings, which bear the brunt of heat re-entry and have reinforced protection.

Even if they did find damage, there was nothing the crew could have done to fix it, he said.

''There's nothing that we can do about tile damage once we get to orbit,'' Dittemore said. ''We can't minimize the heating to the point that it would somehow not require a tile. So once you get to orbit, you're there and you have your tile insulation and that's all you have for protection on the way home from the extreme thermal heating during re-entry.''