Why did the shuttle break up?
Nasa says there is no indication that the tragedy was caused by anything on the ground.
There are already some theories: these include the failure of the heat-protection tiles that shield the shuttle on re-entry.
No-one knows for sure. It will take some time for experts to sift through the evidence.
What was the Columbia shuttle's mission?
It was on a scientific mission to carry out a series of biological, medical and physics experiments.
What will Nasa investigators do now?
Nasa will want to gather all the fragments and piece them together.
It could be a long task - the shuttle was made up of more than a million separate parts.