Space shuttle program manager Ron Dittemore said that after a careful study of the damage possible from the fall of a 20-inch chunk of foam insulation, investigators are "looking somewhere else."
The patch of foam insulation breaking off from the shuttle's external fuel tank during launch and striking tiles on the underside of the left wing has been the leading theory of Columbia's destruction, in which all seven astronauts on board were killed.
In recent days, some space experts have speculated that the chunk of foam was coated or infused with ice, which could have increased the weight — and destructive potential — of the piece that hit the shuttle.
"I don't think it's ice. I don't think there's an embedded ice question here," Dittemore said, adding that the foam is water-resistant and that an inspection team found no ice conditions that day. "So it is something else.
"It doesn't make sense to us that a piece of debris could be the root cause of the loss of Columbia and its crew," Dittemore said. "There's got to be another reason."
He said investigators are now asking if there was "another event that escaped our attention" that might have caused Columbia to break up just minutes before the end of its 16-day mission. In particular they are concentrating on recovering the last 32 seconds of lost data that my uncover more clues.
Meanwhile, remains of space shuttle Columbia's astronauts arrived at Dover Air Force Base, home of the military's largest mortuary, in seven containers where experts will work to identify them using DNA analysis. A C-141 cargo plane carrying remains that have been found so far arrived from Barksdale Air Force Base in Louisiana.
Earlier on Wednesday, NASA defended the thermal tile system that has come under suspicion in the space shuttle Columbia disaster, saying it has worked well for many flights.
The space agency was warned in a technical report at least nine years ago that the heat-protection tiles on the undersides of the shuttle's wings were susceptible to damage that could destroy the spacecraft.
"We've had a lot of experience with that thermal protection system," said Mike Kostelnik, a top spaceflight official. "We've had good success with the tile system."
He acknowledged, however, that there are key sites on the underside of the wing, such as the covers of the landing gear compartment, that make the craft vulnerable during re-entry into Earth's atmosphere.
"They would be a source of problems if they are not properly sealed," Kostelnik said.
A technical report published in 1994 warned the space shuttle could be destroyed if tiles protecting critical wing parts were damaged by debris during liftoff, but NASA engineers never found a complete solution for the safety soft spot.
NASA struggled for years in trying to ensure that the tiles were firmly attached to the shuttle, Paul Fischbeck, an engineering professor at Carnegie Mellon University, said in his analysis.
He said Tuesday that NASA engineers "took a lot of our advice to heart" and made changes to lower the risk of debris hitting the tiles during launch. But the problems were never completely solved, he said.
"There are very important tiles under there. If you lose the tiles on those stretches ... it can cause the shuttle to be lost," Fischbeck said.
As Columbia neared its tragic end, sensors picked up heat spikes on the left side, and the shuttle began pulling strongly to the left: a scenario consistent with significant tile damage, reports CBS News Correspondent Bob Orr. Rough, missing, or uneven tile under the wing could have so disturbed the airflow. Aerodynamic forces may have torn the orbiter apart.
"If the drag force on the left hand side were large enough they could have lost attitude control of the vehicle and it would have turned broadside into the wind,and this would have precipitated its rapid breakup," Edward Crawley of MIT told CBS News
While the prime focus remains on the tiles, a number of other possible causes are also being explored. CBS News Correspondent Barry Bagnato reports a top NASA official tells the Los Angeles Times investigators are considering whether the shuttle was struck during orbit by some piece of space junk.
Meanwhile, the search for parts of the shattered Columbia was expanded westward to California and Arizona, where teams were checking reports of debris. That material could provide clues to the earliest stages of Columbia's disintegration.
Recovery teams sent a helicopter Wednesday to hoist a crucial piece of wreckage — a nose cone section — from an east Texas forest. Sabine County Sheriff Tom Maddox said an aerial search of the region had also turned up signs of several objects in a lake.
Investigators desperately need to find the parts of the shuttle, which fell off first, to understand what caused the spacecraft to break up over Texas, reports Orr.
Law enforcement officials also said they were taking action against looters, after days of warning people to stay away from debris.
Fischbeck and his colleagues made an initial report to NASA in 1990 and published a follow-up paper in 1994.
They conducted a risk analysis of the shuttle's thermal protection tile system and found the spacecraft was highly vulnerable to tiles being knocked off or broken by insulation falling from the fuel tank and from other debris.
The analysis found that the most vulnerable shuttle parts were the undersides of the wings close to the fuselage and right under the crew compartment.
NASA experts said that data from Columbia shows a sudden temperature rise — a marker for failed tiles — in the left wheel well, an area Fischbeck's report said was a critical risk.
Kostelnik said that foam insulation has peeled off during earlier launches, but none was the size of the chunk that went sailing off Columbia's fuel tank.
Fischbeck said NASA has improved the protection and maintenance of the tile system since his study. Foam insulation on the fuel tank was changed, and there are stricter limits on how much ice is allowed on the fuel tank before launch.