Space Shuttle Columbia blew up and disintegrated in flames 39 miles over Texas Saturday morning, killing all seven astronauts aboard and scattering debris over up to four states and the Gulf of Mexico.
The sight of the shuttle breaking up above the Earth and sending a meteoric streak of debris across the sky was horrifyingly reminiscent of the Challenger disaster almost exactly 17 years ago to the day.
The seven crew members -- six Americans and the first Israeli to go into space -- were scheduled to touch down in just 16 minutes at Cape Canaveral, Fla., when the shuttle broke up at 207,135 feet. The astronauts had been orbiting the Earth for 16 days.
"Columbia is lost. There are no survivors," President Bush said in a televised address from the Cabinet Room. He said the day had brought "terrible news" and "great sadness" to the country, and that "our entire nation grieves."
The president ordered flags to be lowered to half-staff at all government buildings.
An independent commission was appointed to investigate the cause of the tragedy, which was not immediately known.
The probe focused immediately on possible damage to the protective thermal tiles on the left wing of the shuttle. A piece of insulating foam broke off from the external fuel tank during the Jan. 16 liftoff and may have knocked against the 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.
NASA said the first indication of trouble Saturday was the loss of temperature sensors in that wing's hydraulic system.
The spacecraft had just re-entered the atmosphere and had reached the point at which it was subjected to the highest temperatures.
Authorities said there was no indication of terrorism; at 207,135 feet, the shuttle was out of range of any surface-to-air missile, a senior government official said. Security was extraordinarily tight on the mission because Ilan Ramon, Israel's first astronaut, was among the crew members.
Television footage showed a bright light followed by white smoke plumes streaking diagonally through the brilliant, blue sky. Debris appeared to break off into separate balls of light as it continued downward.
Military satellites with infrared detectors recorded several flashes as Columbia broke apart, according to a defense official who spoke 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.
"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."
Pieces of the spacecraft were found in several east Texas counties and in Louisiana. There also were unsubstantiated reports of debris in New Mexico and Arkansas. Among the items found: An astronaut's charred patch, and a flight helmet.
There was at least one report of human remains recovered. In Hemphill, Texas, near the Louisiana line, a hospital employee on his way to work reported finding what appeared to be a charred torso, thigh bone and skull on a rural road near what was believed to be other debris.
The FAA issued a notice to airmen because the National Weather Service radar picked up a debris cloud about 95 miles long and 13 to 22 miles wide over Lake Charles, La.
The Army's 1st Cavalry Division sent a helicopter search-and-rescue task force from Fort Hood, Texas. NASA also asked members of the public to help in its search for debris, but warned people not to touch the pieces because they might be contaminated with toxic propellants.
The shuttle flight was the 113th in the shuttle program's 22 years and the 28th flight for Columbia, NASA oldest shuttle.
The horrific end of shuttle mission STS-107 was a devastating blow to the nation's space program; the Challenger explosion led to a 2-year moratorium on launches, and Saturday's accident could bring construction of the international space station to a standstill.
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.
The loss of the seven astronauts -- 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, and to a NASA program that will never forget the 1986 Challenger disaster.
"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 no indication of any trouble.
Mission Control radioed: "Columbia, Houston, we see your tire pressure messages and we did not copy your last."
Columbia's commander, Rick Husband, calmly responded: "Roger, buh ..."
For several seconds, the transmission went silent.
Then, there was 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.
"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."
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.
Jeff Hancock, a dentist, said a metal bracket about a foot long had crashed through his office roof.
"It's all over Nacogdoches," said James Milford, owner of a downtown barber shop. "There are several little pieces, some parts of machinery. ... There's been a lot of pieces about 3 feet wide."
In 42 years of U.S. human spaceflight, there had never been an accident during descent 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 southwestern 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.
Former astronaut John Glenn, the first American to orbit the Earth, and his wife were watching on television at their home in Maryland.
"Anytime you lose contact like that, there's some big problem. Of course, once you went for several minutes without any contact, you knew something was terribly wrong," Glenn said.
The shuttle is essentially a glider during the hour-long descent 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.
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 -- 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 government of Israel and the people of Israel are praying together with the entire world for the safety of the astronauts on the shuttle Columbia," Prime Minister Ariel Sharon's office said in a statement. "The state of Israel and its citizens are as one at this difficult time."
Dr. Yael Barr of the Israeli Aerospace Medicine Institute was waiting at the landing strip for the astronauts' return.
"When the countdown clock, when it got to zero and then started going, instead of counting down, counting up and they were still not there, I told my friend, 'I have a bad feeling. I think they are gone.' And I was in tears," Barr said.
Just in the past week, NASA observed the anniversary of its only two other space tragedies, the Challenger explosion on Jan. 28, 1986, and the Apollo spacecraft fire that killed three on Jan. 27, 1967.
The Associated Press contributed to this report.