Glitch in the Columbia's computer system and differential temperature in the left wing of the shuttle are the areas that investigators will focus on to determine the cause of its break-up on Saturday.

As the shuttle took off, a piece of foam insulation had broken off from a fuel tank and hit the left wing. It was not carrying any robot arm and hence it was impossible to look under the wing for any damage.

But apparently, from the video of the takeoff, it was concluded that there was not much damage.

Experts are now of the opinion that if whatever hit the left wing damaged the tiles, which reflect heat at the time of re-entry, things could have turned sour.

At the last stages of the re-entry, the shuttle is under the control of the computers, which have a lot redundancy built into it.

Meanwhile, the Russian space agency has offered to help determine the causes of the crash.

Viacheslav Mikailichenko, one of the directors of the Russian agency, told Itar Tass news agency that the investigation would be difficult because of the high altitude at which the shuttle disintegrated -- leaving only small fragments of debris.

"The Russian space agency extends its condolences to its American counterpart," Mikailichenko said. "It's a terrible catastrophe."

"Judging by the trajectory of the debris it was a disintegration," Mikailichenko said. "When there's an explosion the debris fragments fly off in all directions."

He said the cause of the disaster was likely to be metal fatigue in the 22-year-old shuttle or the loss of some of the heatproof ceramic plates designed to protect it against the heat generated due to the high-speed re-entry into the atmosphere.