The US space agency has begun an internal and external, independent investigation into the loss of the orbiter Columbia. These are the facts as they are understood.
It is a perfect day to land the shuttle. There are no weather issues to complicate the orbiter's final approach to the Kennedy Space Center in Florida. The touchdown is set for 0916 EST (1416 GMT).
Columbia begins its de-orbit burn at 0815. The shuttle's 16-day science mission is over and it is heading home.
At 0852, the first indications of problems begin to emerge. Three brake-line sensors in the left wheel well show an unusual temperature rise.
At 0853, there is a loss of temperature sensors in hydraulics systems in the trailing edge of the left wing. Temperature sensors in the left wheel well show an abnormal rise - 30-40 degrees - in five minutes. The shuttle is currently over California.
At 0854, as Columbia moves over Nevada, sensors on the main fuselage above the left wing show a temperature rise of 60 degrees.
At 0855, yet another sensor in the left wheel well shows a temperature rise.
At 0856, sensors in the left main gear tyre-wheel continue to report a temperature increase.
At 0857, skin temperature sensors fail.
At 0858, the shuttle has reached New Mexico. Three temperature sensors on the left side of the vehicle stop working. At this stage, Columbia is at an altitude of nearly 40 miles; its speed is in excess of Mach 18 (18 times the speed of sound - 13,200 mph). It is still 1,400 miles from the Kennedy runway. Columbia is in a left-bank, with its wings angled about 57 degrees to the horizontal. But the vehicle is also experiencing increased drag on its left side and the automatic flight control systems are trying to correct it.
At 0859, with the vehicle now over west Texas, onboard sensors indicate the left-hand drag is increasing. Two of the four yaw jets on the right side fire for one-and-a-half seconds to try to correct the vehicle's attitude.
Sensors monitoring tyre temperatures and pressures report no data. Mission control contacts the shuttle: "Columbia, Houston. We see your tyre-pressure messages. We did not copy your last."
There is a very short delay before the shuttle comes back: "Roger... erm..." The line is lost.
For the next few minutes, repeated calls are made to Columbia - but with no response.
Witnesses in Texas describe hearing a huge bang and see several white trails in the blue sky.
At 0929, Nasa puts into practice its contingency action plan. President Bush is telephoned at Camp David to inform him that the shuttle is lost.
Rescue units are scrambled to search in Texas for the shuttle and its seven astronauts. Local emergency services are asked for assistance.
A Mishap Investigation Team is assembled to probe what went wrong. The order is sent to preserve all computer data relating to the last few minutes of the flight. The order is also sent to secure all pre-flight inspection and servicing records for Columbia.
Efforts also begin to set up an external Mishap Investigation Team. It will contain personnel from the US Airforce, Navy and other agencies.
Nasa Administrator Sean O'Keefe speaks with the families of the astronauts lost on Columbia.
President Bush also calls the families of the astronauts lost on Columbia to offer his sympathy and to promise a full investigation.
The Texas public safety department reports more than 2,000 debris fields, scattered from the small town of Nacogdoches, about 170 miles south-east of Dallas, to the Louisiana border.
The track Columbia should have taken over the US