A mishap during takeoff is emerging as the leading theory to the cause of Columbia's catastrophic disintegration on Saturday.

NASA engineers are taking a long, hard look at the possibility that damage to space shuttle Columbia's thermal tiles may have caused the spacecraft to break up while re-entering the Earth's atmosphere Saturday, killing all seven astronauts aboard.

The engineers are closely examining the possible effect of a broken-off piece of insulation that hit the left wing of the shuttle during its launch Jan. 16, said Bill Readdy, an associate administrator.

"Everyone has leaped to the conclusion that was the cause," Readdy said. "That is certainly the leading candidate right now, but we have to rule things out."

Readdy said an engineering report issued during the 12th day of Columbia's 16-day flight concluded that even if the tiles were damaged, there would be "no burn-through and no safety-of-flight issues."

The tiles are designed to keep the 3,000 degrees of heat generated during re-entry from reaching and melting the aluminum hull of the space shuttle.

Retired Air Force Gen. Mike Kostelnik, a deputy associate administrator for NASA, said that the possible effect of the insulation was thoroughly investigated by "the best and brightest" engineers at NASA, who concluded that it was not a safety risk.

Both Kostelnik and Readdy said they saw the engineering report and accepted the assessment.

"We're very aware of the anomaly that was observed," Kostelnik said. He said he "trusted implicitly" the expertise of the engineers and "we were in complete agreement with their assessment."

The report said that a foam insulation patch about 7 inches by 30 inches in size popped off the fuel tank about 80 seconds after Columbia had left its launch pad, Readdy said. NASA engineers spotted the peeling insulation on high-speed cameras that recorded Columbia's launch.

Meanwhile, NASA expanded the search for debris Monday, establishing a second collection site at the former Carswell Air Force Base in Fort Worth. The search area now stretches from central Texas to western Louisiana.

NASA officials said that teams are collecting and cataloging thousands of pieces and taking them to the Fort Worth base and to Barksdale Air Force Base in Louisiana, where investigators are settling into the long task of figuring out why Columbia broke apart.

Carswell closed a decade ago and later reopened as the Naval Air Station Fort Worth/Joint Reserve Base.

• Map: Debris Sightings

Kostelnik said one emphasis is the recovery of the remains of the seven astronauts, mostly in the Lufkin, Texas, area.

"We are trying to recover these national heroes and get them back to their families as soon as possible," Kostelnik said. A number of body parts already have been recovered.

The shuttle is covered by more than 20,000 tiles that keep it from burning up during re-entry into the atmosphere.

New evidence released Sunday 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.

NASA shuttle program manager Ron 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. Then there was 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.

Despite the possible clues disclosed on Sunday, Dittemore stressed 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 insulation "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, the widow of shuttle commander Rick Husband, on NBC's Today.

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.

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."

The Associated Press contributed to this report.