U.S. President George W. Bush Tuesday led the nation in mourning the loss of the seven astronauts who died on board the space shuttle Columbia. Mr. Bush said U.S. space exploration will continue.

President Bush said each of the lost astronauts knew that the great endeavors of space exploration are inseparable from great risk, and yet they accepted those risks willingly, even joyfully, in the cause of discovery.

"Their mission was almost complete. And we lost them so close to home," he said. "The men and women of the Columbia had journeyed more than six million miles and were minutes away from arrival and reunion," he said. "The loss was sudden and terrible, and for their families, the grief is heavy."

The president told families at the Johnson Space Flight Center in Texas that the nation shares in their sorrow and in their pride. Mr. Bush said America remembers not only one moment of tragedy but seven lives of great purpose and achievement.

As he did immediately following Saturday's disaster, President Bush again vowed that U.S. space exploration will continue.

"The people of NASA are being tested once again," he said. "In your grief, you are responding as your friends would have wished, with focus, professionalism, and unbroken faith in the mission of this agency."

NASA Administrator Sean O'Keefe said the space agency will investigate the cause of the shuttle's break-up and will correct what went wrong with what he called "an uncompromising commitment to safety." Mr. O'Keefe stressed that is an obligation owed the American people as well as the families of those who died.

"As we continue to pursue our mission goals of understanding and protecting our home planet, exploring the universe and searching for life, and inspiring the next generation of explorers, we hope our unceasing efforts will provide a fitting tribute to the Columbia seven," he said.

Investigators are focusing on computer readouts showing a sudden increase in temperature on Columbia's left side, shortly before the disaster. They are also re-evaluating earlier determinations that fuel tank debris from lift-off had not done significant damage to insulating tiles.