In its first significant determination, the accident investigation board announced that heat damage from a missing tile would not be sufficient to cause unusual temperature increases inside Columbia minutes before it disintegrated. Sensors detected an unusual heat buildup of about 30 degrees inside the wheel well before the accident.
The board determined those increases were caused by the presence inside Columbia of plasma, or superheated air, with a temperature of roughly 2,000 degrees. It said investigators were studying where a breach might have occurred to allow plasma to seep inside the wheel compartment or elsewhere in Columbia's left wing.
The announcement focused renewed attention on possible catastrophic failures inside the wheel compartment that may have attributed to the Feb. 1 breakup that killed seven astronauts.
Officials are not sure where a hole might have opened in Columbia's skin, NASA spokesman James Hartsfield said. But he said the leading edge or elsewhere on the left wing, the fuselage or the left landing gear door were prime candidates.
"Any of those could be potential causes for the temperature change we saw," Hartsfield said. "They do not and have not pinpointed any general location as to where that plasma flow would have to originate."
The board dismissed suggestions Columbia's left landing gear was improperly lowered as it raced through Earth's atmosphere at more than 12,000 miles per hour. NASA disclosed earlier Thursday that a sensor indicated the gear was down just 26 seconds before Columbia's destruction.
If Columbia's gear was lowered at that speed — and in those searing temperatures as the shuttle descended over Texas from about 40 miles up — the heat and rushing air would have sheared off Columbia's tires and led quickly to the spacecraft's tumbling destruction, experts said.
Officials said they were confident that unusual sensor reading was wrong. Tires are supposed to remain raised until the shuttle is about 200 feet over the runway and flying 345 miles per hour.
Two other sensors in the same wheel compartment indicated the gear was still properly raised, they said.
While Columbia's piloting computers began almost simultaneously firing thrusters struggling to keep wings level, officials said a mysterious disruption in the air flowing near the left wing was not serious enough to suggest the shuttle's gear might be down.
The investigating board concluded that its research "does not support the scenario of an early deployment of the left gear."
Meanwhile investigators still believe important clues might be found in west Texas and points even farther west — even though no debris has yet been found.
The reason for their faith in the absence of evidence is a wealth of credible photographs, video recordings and eyewitness reports from California, Nevada, Arizona and New Mexico. More than 1,500 photographs and videos of Columbia's re-entry have poured in to NASA.
But so far, NASA has ruled out more than 100 of the 179 credible reports of possible debris found west of Fort Worth. And the agency has no data indicating it was already in trouble as it made landfall after crossing the Pacific Ocean.
NASA also confirmed that searchers in Texas recovered what is believed to be one of Columbia's radial tires. A spokesman was not immediately sure which of the shuttle's six tires was found.
The tire was blackened and sustained a massive split across its tread, but it was impossible to know whether the tire was damaged aboard Columbia or when it struck the ground.
The board's announcement came one day after NASA released e-mails showing midlevel safety engineers in Virginia and Houston considered the risks of tires bursting inside Columbia's belly from heat damage.
Robert H. Daugherty, responding to an inquiry from Johnson Space Center, cautioned in one of those e-mails that damage to insulating tiles near the landing gear door could cause one or more tires inside to rupture, perhaps ending with "catastrophic" failures that would place the seven astronauts "in a world of hurt."
Milt Heflin, chief of the flight director's office, said Daugherty and others involved in the tire questions "were happy with the analysis and the work that was done" by Boeing. "They were continuing to do more what-if'ing."
Heflin added that that is a common practice.
However, reports CBS News Correspondent Peter King, Heflin couldn't say whether anyone at NASA knew of both the e-mail messages and a Boeing study on the protective tiles that said losing some tiles in that area of the shuttle wouldn't be a flight safety issue.
Ret. Admiral Harold Gehman, who heads the panel investigating the Columbia accident, called Daugherty's concerns "one of the many, many interesting leads that we have."
Also Thursday, remains of all seven Columbia crew members have been positively identified, and the search for additional remains has ended, NASA officials said Thursday.
The identifications were made at the military mortuary at Dover Air Force Base, where the remains were taken after the space shuttle broke apart during re-entry Feb. 1.
"We are comforted by the knowledge we have brought our seven friends home," said Bob Cabana, director of Flight Crew Operations at the Johnson Space Center, in a prepared statement. "We are deeply indebted to the communities and volunteers who made this homecoming possible, and brought peace of mind to the crew's families, and the entire NASA family."
"We are working toward releasing the crew remains to the families for their own private memorial services," said Eileen Hawley, a spokeswoman for the Johnson Space Center in Houston.