Why did the Space Shuttle Columbia disintegrate during its re-entry into Earth's atmosphere?

Explanation

The Space Shuttle Columbia was one of the five first US Space Shuttles. It was first sent into space on 2 April 1981, on the 20th anniversary of Gargarin's space flight. It disintegrated on its 28th mission on its re-entry into Earth's atmosphere on 1 February 2003. All seven crew members were killed because a piece of insulation material from the fuel tank peeled off during take-off, damaging the heat shield on the wing.