NASA says new evidence shows that the temperature on Columbia's left side shot up and the ship was buffeted by greater wind resistance before it disintegrated over Texas, killing all seven astronauts aboard. Those conditions forced its automatic pilot to quickly change course.
The combination of these events suggests that thermal tiles may have been damaged during launch by a loose piece of foam insulation from the shuttle's external fuel tank. The shuttle's exterior is covered with thousands of tiles designed to protect it from the extreme heat of re-entry.
Despite the possible clues, shuttle program manager Ron Dittemore stressed Sunday that the information was only preliminary.
"We've got some more detective work," Dittemore said. "But we're making progress inch by inch."
NASA Administrator Sean O'Keefe also stressed that other theories couldn't be ruled out yet.
The foam "is one item of many, many pieces of evidence we're collecting in an effort to try to determine the cause of this accident," O'Keefe said Monday on CBS' "The Early Show." "We're not ruling anything out and that is not a favored theory at this point."
The families of Columbia's crew members said Monday they want their loved ones' legacy to continue.
"Although we grieve deeply, as do the families of Apollo I and Challenger before us, the bold exploration of space must go on. Once the root cause of this tragedy is found and corrected, the legacy of Columbia must carry on for the benefit of our children and yours," they said in a statement read by Evelyn Husband, wife of shuttle Cmdr. Rick Husband, on NBC's "Today."
While engineers at the Johnson Space Center in Houston analyzed billions of bits of electronic data radioed to Earth by Columbia on Saturday morning, state and federal officials collected bits and pieces of the shattered spacecraft over a broad swath of east Texas and Louisiana.
The debris was being catalogued and trucked to an Air Force base in Louisiana. Some human remains also have been recovered.
President Bush scheduled a meeting Monday with O'Keefe to get an update on the disaster.
Computer data indicates that moments before Columbia broke apart on Saturday on its way toward a landing in Florida, temperatures rose in the wheel well and on the fuselage on the left side of the shuttle. That was the same side of the craft that was hit by the fuel-tank insulation during the craft's Jan. 16 launch, NASA engineers said.
Dittemore said engineers also planned to examine 32 seconds of computer data that earlier had been ignored because it was considered flawed. The data came just before all communications with Columbia were lost.
NASA engineers spotted the peeling insulation on high speed cameras that recorded Columbia's launch. Dittemore said the possible effects on the tiles from the insulation were studied aggressively while the shuttle was still aloft, but engineers concluded "it did not represent a safety concern."
"As we gather more evidence, certainly the evidence may take us in another direction," he said.
NASA's best estimate is the piece of foam was up to 20 inches long, spokesman Allard Beutel said Monday.
Dittemore said engineering data shows a rise of 20 to 30 degrees in the left wheel well about seven minutes before the spacecraft's last radio transmission. There followed a rise of about 60 degrees over five minutes in the left side of the fuselage above the wing, he said.
The right side of the shuttle rose the normal 15 degrees over the same period, he said. All the readings came from sensors underneath the thermal tiles, on the aluminum hull of the craft.
The temperature spikes were accompanied by an increased drag, or wind resistance, that forced Columbia's automated flight control system to make rapid adjustments maintain stability. Dittemore said the corrections were the largest ever for a shuttle re-entry, but still within the craft's capability.
Lockheed, the maker of the fuel tank under scrutiny, said Sunday that NASA used an older version of the tank, which the space agency began phasing out in 2000. NASA's preflight press information had said the shuttle was using one of the newer super-lightweight fuel tanks.
Harry Wadsworth, a spokesman for Lockheed, said most shuttle launches use the "super-lightweight" tank and the older version is no longer made. Wadsworth said he did not know if there was a difference in how insulation was installed on the two types of tanks.
Wadsworth said the tank used for the Columbia mission was manufactured in November 2000 and delivered to NASA the next month. Only one more of the older tanks is left, he said.
Dittemore said the older version of the tank had been used for many years and was 6,000 to 7,000 pounds heavier than the newer version. Still, "we had no reason to doubt it capability."
Earlier Sunday, O'Keefe named a former Navy admiral to oversee an independent review of the accident, and said investigators initially would focus on whether the piece of insulation caused the damage that brought down the shuttle.
While O'Keefe stressed that the space agency was not locking into a single scenario of what caused the crash, the insulation was "one of the areas we're looking at first, early, to make sure that the investigative team is concentrating on that theory."
Meanwhile, searchers using horses and four-wheel-drive vehicles scoured rural areas of east Texas and western Louisiana for bits of metal, ceramic tile, computer chips and insulation from the shattered spacecraft.
State and federal officials, treating the investigation like a multi-county crime scene, were protecting the debris until it can be catalogued, carefully collected and then taken to Barksdale Air Force Base in Louisiana.
The effort to reconstruct what is left of Columbia into a rough outline of the shuttle will be tedious and painstaking.
When a shuttle piece was located this weekend, searchers left it in place until a precise global positioning satellite reading could be taken. Each shuttle part is numbered; NASA officials say experts hope to trace the falling path of each recovered piece.
The goal is to establish a sequence of how parts were ripped off Columbia as it disintegrated under the intense heat and pressure of the re-entry into the atmosphere.
In addition to the search on land, divers were being called in to search Toledo Bend Reservoir, on the Texas-Louisiana line, for a car-sized piece seen slamming into the water.
Some body parts from the crew have been recovered and may be sent to a military morgue at Dover Air Force Base in Delaware.