NASA investigators seeking the cause of Saturday's shuttle disaster are taking another hard look at a sheet of insulating foam that broke away from Columbia's external fuel tank and struck the craft's left wing during liftoff Jan. 16.

Engineers had dismissed the danger from the flying insulation while the astronauts were in orbit. But Ron Dittemore, the shuttle program manager, said during a news conference in Houston yesterday that it remains the best lead in the mysterious disaster that claimed the lives of seven crew members.

"We're making the assumption that the external fuel tank was the root cause of the problem that lost Columbia," he said. "We'll see where that leads us."

As he spoke, scientists and engineers were sifting through more than 12,000 pieces of shuttle debris that have been recovered in Texas and Louisiana. Yesterday's discoveries included a piece of the cabin segment and nose cone.

Dittemore said the most important clues are likely to come from any debris found further "upstream" in Arizona or New Mexico. Those would be the first pieces that broke from the craft and a clue to the source of the fatal disintegration.

Meanwhile, scientists will rerun their analysis of what happened when a 2.7-pound sheet of hardened foam peeled off the shuttle's external fuel tank 80 seconds into liftoff, striking the underside of the wing and vaporizing in a puff of tiny pieces of debris.

Caught during a review of film the day after liftoff, it raised concerns about possible damage to the shuttle's critical tiles, which protect the craft against heat that reaches 3,000 degrees on re-entry.

Engineers began an intensive study that concluded on the 12th day of the mission. Dittemore said the team decided that even if the heat shield tiles had been damaged, it would not pose "a flight safety issue."

"At the time, I was not aware of reservations by any individual on our team," he said, noting that the computer simulation used to analyze the incident was well-tested and conservative.

Columbia, the oldest of the National Aeronautics and Space Administration's four shuttles, was destroyed 39 miles above Texas on a landing approach that was to end at Cape Canaveral, Fla. after a successful 16-day scientific mission.

NASA's investigation revealed yesterday that in Columbia's final seconds, its flight computers were in a losing struggle to keep the spacecraft from making an uncontrolled turn to the left.

NASA officials said increasing wind resistance pulling the shuttle to the left caused the shuttle's four right "yaw" jets to fire to assist other flight controls wrestling with the problem.

"It appears we were losing ground," said Dittemore. "It was not long after that point that we lost all data and communications with the crew."

NASA officials planned to take time today for a memorial service for the seven lost Columbia astronauts. President Bush was to attend services at the Johnson Space Center.

There was to be no other interruption in the investigation or the search for the remains of the shuttle and its crew.

In Nacogdoches County, Texas, Sheriff Thomas Kerss said searchers had found what he described as 7-foot piece of the shuttle crew cabin and computer components.

Part of the shuttle's nose was found buried deep in a 20-foot-wide hole on the Texas-Louisiana border.

The debris and crew remains were being collected at Barksdale Air Force Base near Shreveport, La., and at Carswell Air Force base near Fort Worth, Texas.

Dittemore said yesterday that barely 1 percent of the shuttle's bulk had been recovered, none of it of critical importance. Intact, Columbia weighed about 180,000 pounds, but none of the debris struck or injured anyone.