The investigation into catastrophic loss of the space shuttle Columbia and its crew of seven on Saturday morning is narrowing in on the spacecraft’s left side, which became abnormally hot just before contact was broken with mission control.

NASA stresses that the cause of the tragedy remains unknown. But the most probable explanation based on current knowledge is that heat-resistant tiles on the underside of the shuttle’s left wing were significantly damaged by debris during take off. This could have caused the intense frictional heat of reentry to the Earth’s atmosphere to penetrate the shield and weaken the wing. Travelling at about 20,000 km/h, the shuttle would have rapidly disintegrated.