The U.S. space agency, NASA, has mobilized search and rescue teams after the space shuttle Columbia apparently disintegrated in flames over Texas, minutes before it was due to land in Florida. There were seven crew members on board when disaster struck.
NASA has lowered the flag next to its countdown clock in Florida to half-staff.
The U.S. space agency declared an emergency after communication was lost Saturday morning with the space shuttle Columbia - which was carrying six Americans and the first Israeli astronaut, Ilan Ramon.
Television video of the shuttle re-entering the earth's atmosphere showed several white smoke trails, indicating the shuttle may have broken into pieces.
NASA mobilized search and rescue crews in Texas to search for any wreckage. There was no immediate word on what may have caused the breakup.
President Bush left his weekend retreat at Camp David outside Washington to meet with senior administration staff at the White House.
Security had been tighter than usual at the landing site because of the Israeli astronaut's presence. But Homeland Security spokesman Gordon Johndroe says, there is no indication that terrorism could have been involved.
In Israel, people watched in shocked disbelief, as national television interrupted regular programming to carry images of the catastrophe. The flight was a source of national pride for Israel, because it was carrying an Israeli astronaut.
Prime Minister Ariel Sharon's office issued a statement saying the government and people of Israel were praying for the safety of the astronauts on the shuttle, and awaiting further word on their fate.
At 22-years-old, Columbia is the oldest shuttle of the fleet. It was returning from a 16-day scientific mission, where astronauts carried out more than 80 experiments.
This is the second disaster involving a space shuttle. In 1986, a gas leak in one of the rockets caused the space shuttle Challenger to explode shortly after take off, killing all seven crew members on board.