As the days become weeks since Columbia's disintegration over Texas, fewer and fewer pieces of space shuttle 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 for anything that might have fallen from the sky on Feb. 1. The plea came as a reported 1,300 state and federal personnel took part in search and recovery efforts in Texas and Louisiana.

"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.

He added that "there's a continuing belief and feeling that things are going to continue to develop" and that more debris may be found.

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.

Nesbitt took a phone call over the weekend from a Louisiana woman who has an odd chunk of plastic in her flower bed and wants NASA to check it out. He also heard from a California retiree who wanted to tell the board his theory for the shuttle disaster.

"Everybody wants to contribute. They all want to help and it's great. The board certainly wants to listen," Nesbitt said. But he noted: "It will take a while to get back to the people," given the disproportionate ratio of callers to those following up on all the calls.

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 hold a news conference, their second, Tuesday. 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 forward 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 during liftoff debris hit the vulnerable underside of its wings, near the landing-gear wheel wells.

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.

The head of the independent investigation board, retired Navy Adm. Harold Gehman Jr., has stressed repeatedly that he is not ruling anything out -- including the debris hit during launch. While Columbia was still in orbit, NASA concluded that any damage from the foam would not pose a safety threat.

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.

Despite extensive searches along Columbia's final route, from California down over Nevada, Arizona and New Mexico and finally Texas, no shuttle debris has been found farther west than 20 miles west of Fort Worth. That's where a single thermal tile popped up; NASA still does not know what part of the shuttle it came from.

Seymour Himmel, a retired NASA executive who served for two decades on the Aerospace Safety Advisory Panel, wonders whether any debris that reached the ground "would retain the clues that you need to unravel this mystery."

Columbia was traveling Mach 18, or 18 times the speed of sound, at an altitude of more than 200,000 feet when contact was lost.

"Unfortunately, this isn't like a mystery novel," Himmel said. "In the mystery novels, you get all kinds of clues and in the last 20 minutes, they tell you whodunit. But this is not that kind of a thing. This is a very tedious process."