Space shuttle Columbia disintegrated 39 miles over Texas on Saturday in a meteoric streak that rained smoking debris over hundreds of miles of countryside in at least two states.
All seven astronauts on board were killed, a tragedy that echoed the Challenger explosion almost exactly 17 years earlier.
The catastrophe occurred in the last 16 minutes of the shuttle's 16-day mission, as the craft glided in for a landing in Florida.
"The Columbia is lost," President Bush said in a televised address to the nation, after he telephoned the families of the astronauts to console them.
"The same creator who names the stars also knows the names of the seven souls we mourn today," Mr. Bush said during his speech, his eyes glistening.
Mr. Bush, who had rushed back to the White House from a weekend at Camp David, Md., ordered that the flags at the White House and government facilities around the world be flown at half-staff through next Wednesday.
The cause of the tragedy was not immediately known. An independent commission was appointed to investigate.
Authorities said there was no indication of terrorism. At 207,135 feet, the shuttle was beyond the range of any surface-to-air missile, one senior government official said. Security was extraordinarily tight on this mission because Ilan Ramon, Israel's first astronaut, was among the crew members.
One potential focus of the accident probe: possible damage to protective thermal tiles on Columbia's left wing. NASA said the first indication of trouble Saturday was the loss of temperature sensors in that wing's hydraulic system.
During Columbia's liftoff, a piece of insulating foam from the fuel tank was believed to have hit that wing. Leroy Cain, the lead flight director in Mission Control, assured reporters Friday that engineers had concluded that any damage to the wing was considered minor and posed no safety hazard.
Shuttle program manager Ron Dittemore said the loss of the sensors on the wing was followed seconds later by several other problems, including a loss of tire pressure and indications of excessive structural heating.
CBS News Space Consultant Bill Harwood said possible causes of the crash include a hydraulic problem or a failure of the wing itself. NASA is clearly focusing on the left-wing area of the spacecraft, he added.
"It's a fair bet that looking at the data that they have now secured, they have a pretty good preliminary idea of at least where they need to focus the investigation," says CBS News Transportation Correspondent Bob Orr.
But beyond the data, the investigators face a very challenging problem of trying to collect the parts of the shuttle and go through extensive forensic and metallurgic testing to try to corroborate what the data tells them.
The debris is very badly fragmented. The pieces are very small and spread over a couple of hundred square miles.
Retired CBS News aviation correspondent Eric Engberg, who covered the Challenger tragedy, says all the videotapes taken by amateur photographers along Columbia's flight path "will be gathered up and studied frame by frame. NASA has appealed to everyone with film to get in touch with it. That film is critical."
The phone number to call is (281) 483-3388, which is the Johnson Space Center Emergency Operations center in Houston.
The six Americans and lone Israeli aboard Columbia had been expected to land at Kennedy Space Center in Florida at 9:16 a.m.
At 9 a.m., Mission Control abruptly lost all data and voice contact with the shuttle and crew. At the same time, residents of Texas, Arkansas and Louisiana reported hearing "a big bang" and seeing flames in the sky.
NASA Administrator Sean O'Keefe, his voice breaking at times as he spoke, said, "We trust the prayers of the nation will be with them and with their families," he said. "A more courageous group of people you could not have hoped to know."
The final radio transmission between Mission Control and the shuttle gave no indication of a catastrophic failure.
Mission Control radioed: "Columbia, Houston, we see your tire pressure messages and we did not copy your last."
Columbia's response: "Roger, uh ..."
Then the transmission breaks off.
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 Fahrenheit.
The shuttle was traveling at 12,500 mph, or 18 times the speed of sound, when Mission Control lost all contact and tracking data.
Military satellites with infrared detectors recorded several flashes as Columbia broke apart, according to a defense official who spoke only on condition of anonymity. It was unclear whether those "spikes" of heat indicated an explosion, the burning of pieces of debris re-entering the atmosphere or something else.
Television footage showed a bright light followed by smoke plumes streaking diagonally through the sky. Debris appeared to break off into separate balls of light as it continued downward.
NASA declared an emergency after losing contact with the crew and within minutes said search teams had been sent to the Dallas-Fort Worth area.
Pieces of the spacecraft were found in several east Texas counties and in Louisiana.
There was at least one report of human remains recovered in Hemphill, Texas, near the Louisiana line, where a hospital employee on his way to work reported finding the remains on a rural road near what was believed to be other debris.
In Cherokee County, Texas, CBS newsman Rob Milford watched as a piece of the shuttle's wing was found. "As we flipped it over, you can see the very much burned pieces of the shuttle's tile....it is absolutely scorched black," he reported.
In Nacogdoches, Texas, residents found bits of metal strewn across the city. Dentist Jeff Hancock said a metal bracket about a foot long had crashed through his office roof. At least one ember started a small fire.
Victoria LaFolett, Nacogdoches city manager, told CBS News, "We've had pieces on our runway at the airstrip, we've had pieces in roadways, we've had pieces atop of buildings, we've had it on people's roofs, in yards, and fields. All sorts of sizes and shapes."
NASA warned people not to touch the debris, which could contain toxic chemicals.
In Nacogdoches, Milford says 45-50 people touched the debris and were treated and released at the local hospital. He says over 500 pieces of debris have been found in Nacogdoches County, which covers 975 square miles. Authorities have had 1,500 calls about it.
Troops were being brought in from Ft. Hood, Texas and Ft. Polk, La. to help recover the pieces. Each piece is being treated like it's at a crime scene: roped off, with someone standing over it to guard it, Milford adds.
"We saw it coming across the sky real bright and shiny and all in one piece. We thought it was the sun shining off an airplane," said Doug Ruby, who was driving with his father along a Texas highway, headed for a fishing trip. "Then it broke up in about six pieces — they were all balls of fire — before it went over the tree line."
"The barn started shaking and we ran out and started looking around," said Benjamin Laster of Kemp, Texas. "I saw a puff of vapor and smoke and saw big chunk of material fall."
The shuttle delivers components of the space station to be installed; it also carries crews to and from the station. The three astronauts now on board the station could return to Earth at a moment's notice via a Russian vehicle attached to the space station.
Harwood says the current space station occupants could stay aloft until late spring or very early summer with no visits from shuttles, but then would likely have to use the Russian craft and leave the station unmanned, at least temporarily.
The shuttle flight was the 113th in the shuttle program's 22 years and the 28th flight for Columbia, NASA oldest shuttle.
In 42 years of U.S. human space flight, there had never been an accident during the descent to Earth or landing. On Jan. 28, 1986, space shuttle Challenger exploded shortly after liftoff.
Just in the past week, NASA observed the anniversary of its only two other space tragedies, the Challenger explosion, which killed all seven astronauts on board, and the Apollo spacecraft fire that killed three on Jan. 27, 1967.
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 U.S. flag next to the center's countdown clock was lowered to half-staff.
"A contingency for the space shuttle has been declared," Mission Control somberly repeated over and over.
In another room at Kennedy Space Center, 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 of them had children.
In addition to Ramon, the flight included Indian-born Kalpana Chawla, 41, who became an astronaut in 1994. It was her second trip in space.
Only two other crew members had flown in space before: the shuttle's commander, Rick Husband and Michael Anderson. The four others were rookies: pilot William McCool and David Brown, Laurel Clark and Ramon.
CBS News Space Consultant William Harwood has covered America's space program full time for more than 15 years, focusing on space shuttle operations, planetary exploration and astronomy. Based at the Kennedy Space Center in Florida, Harwood provides up-to-the-minute space reports for CBS News and regularly contributes to Spaceflight Now and The Washington Post.