The loss of seven explorers of space's dark reaches -- shuttle commander Rick Husband, Michael Anderson, David Brown, Kalpana Chawla, Laurel Clark, William McCool and Ramon -- brought a new round of grief to a nation still in mourning after the terrorist attacks of Sept. 11, 2001.
``We trust the prayers of the nation will be with them and with their families. A more courageous group of people you could not have hoped to know,'' said NASA Administrator Sean O'Keefe.
Columbia had been scheduled to land at Kennedy Space Center at 9:16 a.m.
Shuttle program manager Ron Dittemore said ``there was no indication of any impending threats to the vehicle.'' Then there was a loss of data from temperature sensors on the left wing, followed by a loss of data from tire pressure indicators on the left main landing gear.
The final radio transmission between Mission Control and the shuttle, at 9 a.m., gave little indication of any trouble.
Mission Control radios: ``Columbia, Houston, we see your tire pressure messages and we did not copy your last.''
Columbia's commander, Rick Husband, calmly responds: ``Roger, uh, buh ...''
For several seconds, the transmission goes silent.
Then, there is static.
Texas Sen. Kay Bailey Hutchison, in her Dallas neighborhood, said she heard a ``boom, which I thought was the breaking of the sound barrier'' _ and it may have been just that, because the shuttle was traveling at 12,500 mph, 18 times the speed of sound.
Jeff Foreman, an engineer with a physics degree, told CBS News he was taking video and realized quickly something had gone awry.
``When multiple pieces started coming off, I thought that it was highly unusual, that it shouldn't be happening,'' he said. His suspicions were confirmed when he heard seven or eight sonic booms; during typical fly-bys he hears only two.
On the edge of downtown Nacogdoches, 135 miles northeast of Houston, a National Guardsman stood watch over a steel rod with silver bolts that landed in the grass outside a yard. People streamed up to take photos of the debris.
Dentist Jeff Hancock said a metal bracket about a foot long had crashed through his office roof.
In 42 years of U.S. human spaceflight, there had never been an accident during the descent to Earth or landing.
Two hours after the shuttle had been expected to land, the giant screen at the front of Mission Control showed a map of the southwest United States and what should have been Columbia's flight path. The American flag next to the center's countdown clock was lowered to half-staff.
O'Keefe met with the astronauts' families, who had been waiting at the landing site for the shuttle's return. Six of the seven astronauts were married, and five had children.
The shuttle is essentially a glider during the hour-long decent from orbit toward the landing strip. It is covered by about 20,000 thermal tiles to protect against temperatures as high as 3,000 degrees.
Shortly after Columbia lifted off, a piece of insulating foam on its external fuel tank came off and was believed to have hit the left wing of the shuttle. Leroy Cain, the lead flight director in Mission Control, had assured reporters Friday that engineers had concluded any damage to the wing was considered minor and posed no safety hazard.
``As we look at that now in hindsight, we can't discount that there might be a connection,'' Dittemore said Saturday. ``But we have to caution that we can't rush to judgment, because a lot of things in this business that look like the smoking gun but turn out not to be close.''
Columbia's crew had completed 80-plus scientific research experiments during their time in orbit. It was a relatively inexperienced crew; only three -- Husband, Anderson and Chawla 00 had ever flown before.
The others were rookies, including Ramon, the 48-year-old Israeli Air Force colonel. A former fighter pilot who survived two wars, he carried into space a small pencil drawing titled ``Moon Landscape'' by Peter Ginz, a 14-year-old Jewish boy killed at Auschwitz.
``The state of Israel and its citizens are as one at this difficult time,'' Prime Minister Ariel Sharon's office said in a statement.