In a tragic echo of the Challenger disaster, the seven astronauts on the space shuttle Columbia died yesterday when the spacecraft broke apart over Texas just moments before its scheduled landing and disappeared in a trail of fire, smoke and debris.

While families and friends waited expectantly near a Cape Canaveral runway for the end of an otherwise flawless flight, wreckage from the country's oldest shuttle was raining down on startled residents across hundreds of miles of Texas and Louisiana.

"We thought it was the sun shining off an airplane," said Doug Ruby, who described the scene he and his father saw while driving on a Texas road. "Then it broke up in about six pieces. They were all balls of fire before it went over the tree line."

As the crews' shocked families were quickly taken into NASA facilities, news of the disaster swept the country, producing a collective sense of mourning that reminded many of the reaction to the explosion of the Challenger on Jan. 28, 1986.

"You are not alone. Our entire nation grieves with you," President Bush told the astronauts' families in a broadcast to the nation after returning to Washington from Camp David. He said the crew and their families "will always have the respect and gratitude of this country."

At the same time, Bush said, "Our journey into space will go on. In the skies today we saw destruction and tragedy. Yet farther than we can see, there is comfort and hope."

Impromptu memorials sprang up at NASA facilities in Houston and Cape Canaveral, at the National Air and Space Museum in Washington, and at the Kennedy Space Center's astronaut memorial in Florida. Many flags were lowered to half-staff.

Milton Heflin, NASA's chief flight director, was near tears during an afternoon news briefing. Beside him, Ron Dittemore, shuttle program manager, was in a state of disbelief.

"This vehicle had performed flawlessly," Dittemore said. "It's a painful experience to lose our friends, realizing that things had gone so well, and turned out so badly."

Yesterday's accident, the second in-flight disaster in the shuttle's 22-year history, came 17 years and four days after the Challenger exploded during liftoff, also with the loss of all seven crew members.

NASA officials immediately grounded the agency's three remaining shuttles until the cause of the accident is found. Industry experts said the moratorium would likely last at least a year, but less than the 33 months that followed the Challenger accident.

At a briefing in Florida, where he had gone to greet the returning crew and their families, NASA Administrator Sean O'Keefe said somberly that the space agency had ordered preserved all data that might be useful in the investigation. All service on the other shuttles, as well as shuttle-related manufacturing across the country, was halted.

Two investigations

O'Keefe said an independent investigation board, assembled from the Air Force, Navy, Transportation Department and other federal agencies, would be formed to identify the cause of the disaster. NASA also launched a second, internal investigation. The same dual-track process was followed after the Challenger disaster.

Columbia's crew included the first Israeli astronaut, Ilan Ramon, 48, an air force colonel. But O'Keefe said there was no reason to suspect terrorism. Because of Ramon's presence and continuing turmoil in the Middle East, security surrounding Columbia's launch and landing was extremely tight.

The crew also included three U.S. military officers, one of the nation's few black astronauts and a woman who immigrated to America from India.

The crew consisted of Rick Husband, 45, the shuttle's commander and an Air Force colonel; co-pilot William McCool, 41, a Navy commander; Michael Anderson, 43, the payload commander; mission specialists David Brown, 46, a Navy captain; Kalpana Chawla, 41, who was born in India; Laurel Clark, 41, a physician; and Ramon.

NASA investigators immediately rushed to Texas and Louisiana to gather debris, video and still pictures of the shuttle's breakup. The wreckage was scattered over hundreds of miles of countryside. Much of it lay smoldering, sometimes setting grasslands afire.

In Nacogdoches, Texas, Jeff Hancock, a 29-year-old dentist, said he found a chunk of debris in his office. "It came through the roof," he said. "It's about a foot-long metal bracket."