The National Aeronautics and Space Administration on Saturday said that the seven crewmembers of Columbia, including Indian American astronaut Kalpana Chawla, were killed when the shuttle broke up while re-entering the earth's atmosphere.

NASA chief administrator Sean O'Keefe said at the Kennedy Space Centre in Cape Canaveral, Florida: "This is indeed a tragic day for the NASA family; for the families of the astronauts who flew on Flight STS 107 and likewise tragic for the nation."

President George Bush had spoken with the families of the crew "to express deepest national regrets".

He said at this point there is no indication that the mishap was caused by anyone on the ground. He announced that an external group independent from NASA would carry out the investigation.

Bill Readdy, Associate Administrator for Space Flight, said, "Today's events were a very stark reminder that this is a very risky endeavour -- pushing back the frontiers in outer space. After 113 flights, unfortunately people have a tendency to look at it as something that is more or less routine. I can assure you it is not.

"We will find the cause [of the accident], fix it and then move on."

PTI