CHIANG RAI, Thailand The blast that destroyed a Thai Airways jet may have been caused by a malfunction and not a bomb as was initially thought, Thai Prime Minister Thaksin Shinawatra said Sunday. Thaksin and his son were just about to board the Boeing 737-400 on March 3 when the explosion and resulting fire gutted the aircraft as it stood on the tarmac at Bangkok's domestic airport. The premier said an inquiry by the jet's manufacturer had found a fault in the air-conditioning system which it believed may have set off the blast.