Space Center, Houston ? As the days become weeks since Columbia’s disintegration over Texas, fewer and fewer pieces of Columbia wreckage are turning up, even though the calls keep coming in.

On Monday, NASA asked farmers and ranchers out West to be on the lookout during spring plowing or maintenance for anything that might have fallen from the sky on Feb. 1.

“It’s kind of a mixed thing. There’s a tremendous amount of information available already, even though not everything directly points to a particular thing. There are a lot of circumstantial things,” said NASA’s Steve Nesbitt, who is serving as the spokesman for the accident investigation board.

Now that the investigation board is back in Houston following a series of road trips to other NASA centers, the members can settle into a routine and start digging into all of the information being accumulated, Nesbitt said.

Nine of the 10 board members met Monday at their new headquarters in an office building just a mile from Johnson Space Center and planned to have a weekly news conference, their second, today. The 10th member, newly selected Sheila Widnall, a former secretary of the Air Force, will join the group later in the week.

So far, the investigation board has publicly put forth just one hypothesis: that a breach in the left wing likely allowed superheated gases to penetrate the spacecraft.

Paul Fischbeck, a Carnegie Mellon University engineering professor, said that hypothesis makes his own analysis “more and more likely.” In a 1990 study and follow-up research, Fischbeck concluded that a space shuttle could fail catastrophically if debris hit the vulnerable underside of its wings during liftoff.

Barely a minute into Columbia’s flight on Jan. 16, a chunk of insulating foam broke off the external fuel tank and slammed into the bottom of the left wing.

The breach in Columbia could have been caused by a meteor or space debris, or the landing gear compartment door could have been blown open during atmospheric re-entry, Fischbeck said Monday. But the most likely scenario by far, he said, is that the foam damaged or knocked off thermal tiles, more tiles gave way during re-entry and those missing tiles led to a burn-through of the aluminum hull.

During the flight, NASA concluded that the debris strike would not pose a safety threat during re-entry.

The analysis of 32 seconds of additional data collected beyond the loss of communication with Columbia, meanwhile, goes on. The data are of extremely poor quality, yet experts have managed to conclude that an additional two steering jets were firing, for a total of four, in a futile attempt to keep the shuttle on course as it aimed for a Florida landing.

It is “highly unlikely” that the shuttle pilots, rather than the autopilot system, activated those two additional jets before their ship broke apart, said NASA spokesman Rob Navias.