PALESTINE, Texas -- Senior NASA officials toured parts of the space shuttle Columbia debris field in East Texas Wednesday, thanking searchers for their work as the massive recovery operation enters its final weeks.

NASA Administrator Sean O'Keefe and other managers arrived here during the late morning hours and immediately departed for one of the final grids still remaining to be checked off under the scope of the Palestine branch of the Columbia recovery effort.

Teams have been scouring the countryside across East Texas since Columbia broke apart during re-entry February 1, tragically killing all 7 astronauts aboard.

Officials say the Palestine camp is typical of other command posts across the region. U.S. Forest Service employees in the area -- along with workers from other agencies -- represent 45 states.

Joined by U.S. Department of Agriculture Deputy Secretary James Moseley, Chief of the U.S. Forest Service Dale Bosworth, and NASA Associate Administrator for Space Flight Bill Readdy, O'Keefe met with a team returning from a morning full of tedious searching through a portion of a grid that sits on the northern fringe of the primary debris footprint.

Just a handful of the grids -- each spanning about two miles by two miles -- remain to be searched on the ground by an army of people walking almost shoulder-to-shoulder through almost any terrain imaginable, ranging from flat treeless plains to dense thickets that require protective clothing to traverse.

"This is on the southern edge of a northern expansion of about a mile or so, based on what we've seen in analysis that may have picked up additional debris dropping through it," O'Keefe told reporters here. "This ought to be complete, and we ought to complete all that area, within the span of another few weeks...so a little more time to go, but it's starting to wind down here in the weeks ahead."

Other areas of the designated search zone have also been expanded, particularly west of Interstate 45 in Texas, where there is believed to be sparse concentrations of mostly left wing debris. Investigators have traced the start of the problem plaguing Columbia to the left wing leading edge, where a hole or gap began to let super-heated plasma into the wing's interior not long after the shuttle encountered the first traces of Earth's atmosphere.

Radar data, extensive studies of the behavior of objects on ballistic trajectories, and perhaps even trends in the search grids themselves have allowed the agencies involved -- mostly made up of U.S. Forest Service, NASA, Dept. of Agriculture, EPA, and FEMA officials -- to single out areas where certain debris is likely to have fallen. This includes pieces sighted leaving the orbiter over the far western United States and fragments associated with the final breakup of Columbia over Texas.

Even with operations nearing an end, there are still items of great interest to NASA and the Columbia Accident Investigation Board being found with regularity.

"I was walking with one of the guys here, who just told me that earlier this morning they found a piece of the reinforced carbon-carbon leading edge not far from here, and there was a NASA rep there to identify it, and they've already got it shipped off. Every aspect of this is adding more and more to understand this complex process," O'Keefe said.

It is the policy of the coordinating agencies to "fast track" certain pieces that could be instrumental to the investigation. "If you find something that you really get excited about, you need to make a call," said NASA representative Randy Wade.

Cumulatively, about 78 percent of the primary search area has been worked by ground teams. The fruit of these efforts is represented by more than 78,000 pounds of Columbia debris, or 70,000 items, which equals about 37 percent of the orbiter's dry weight.

The Palestine camp is currently scheduled to close around April 23, followed by the Hemphill command post on April 25, Nacogdoches around May 2, and Corsicana around May 5. The field office in Lufkin responsible for the overall control of the effort could shut down in early May. A Columbia Recovery Office is being set up at Houston's Johnson Space Center to take the helm for the foreseeable future, NASA said in a statement.