The chairman of the independent board investigating the loss of the space shuttle Columbia today promised a final report that will go beyond the disaster's cause and frame the debate on the future of manned space flight itself.

Yet even as retired Adm. Harold W. Gehman Jr., chairman of the Columbia Accident Investigation Board, laid out his bold vision, he acknowledged that only scant physical evidence of the fallen orbiter has been discovered so far in the dense, piney woods of east Texas and Louisiana, leaving investigators with few material clues.

"It is our goal as a board, that no matter what we find here, that the report we write will be deep enough and rich enough that it will be the foundation for a good intellectual debate about what we do next," he said.

While he suggested the board would stop short of a recommendation on whether to continue or end the space shuttle program, he said: "Our report will not be an individual random data point on a graph. Our report will place this event in context of our space exploration quest and attempt to put it in its rightful place."

In the investigation board's second formal news conference, a stark dichotomy emerged between the broad scope of the panel's ambitions and the paucity of evidence from the space shuttle itself, which investigators say is crucial.

Although 3,656 pieces of Columbia have arrived at the Kennedy Space Center in Florida, and perhaps 10,000 more are en route, board members said they amount to no more than 5 percent -- and possibly as little as 1 percent or 2 percent -- of the total reentry weight of the 117-ton Columbia.

Moreover, no critical pieces of evidence, and no patterns, have emerged from the wreckage.

"Right now we have a tiny, tiny portion of the orbiter," said Maj. Gen. John Barry, who leads the investigation board's team focusing on materiel and management matters.

The biggest problem for investigators has been the wide swath of the southwestern corner of the nation that the shuttle traveled as it began to break apart.

Board member James Hallock, who heads the Transportation Department's aviation safety section, said the commission has been studying films of the shuttle's reentry, and "it does look like things were starting to come from the shuttle as it approached California."

Experts are now trying to extrapolate where the wreckage would have landed, but commission members said they expect it to be difficult to find. "We have the Grand Canyon area, all the areas of Southern California, mountainous areas -- it's going be to very difficult to find, but I sure would like to see it," Hallock said.

Board members again asked citizens to report any wreckage found, and stressed the importance of reassembling the pieces at Cape Canaveral.

"We still need debris," Gehman said. "Debris collection is extraordinarily important to us."

He said that about 2,600 pieces of the space shuttle had been identified, catalogued and laid out on the "reconstruction floor" of a hangar, and more than 1,000 more were being processed at the space center. Thousands more pieces are being collected, processed and transported, first to Barksdale Air Force Base in Louisiana, where NASA has established a staging area, and then to the space center in Florida.

Meanwhile, Gehman and NASA Administrator Sean O'Keefe took pains today to stress the investigation board's independence in an attempt to mollify congressional critics who have complained that the panel remains too close to the space agency.

Gehman said the commission is replacing its NASA start-up staff, including the commission's spokesman, with independently hired staff. The commission staff, now numbering about two dozen, is expected to triple in the coming weeks, he said.

Moreover, Gehman said the board would hold public hearings -- the first a week from Thursday -- in which experts "who are not associated with any U.S. government program" will be invited to present their "theories and hypotheses" about the causes that led to the loss of Columbia.

He also said the board is establishing a toll-free phone number and a Web site through which individuals can forward information to the board without going through NASA.

In Washington, O'Keefe announced the third set of revisions of the commission's charter in little more than a week. The changes have eliminated any reference to NASA overseeing or reviewing the commission's work, or setting a 60-day deadline for the investigators to complete their work.

O'Keefe also dropped a requirement that the panel submit its final report directly to him. Instead, the board may provide a final written report "at such time and in such manner as the Board deems appropriate." Upon its completion, the report will be immediately released to the public. The archives, which Gehman said may include about 10,000 documents, would also be available to the public.

The latest changes removed any suggestion that the board would have to use NASA support staff in such areas as public affairs or legal or security matters.

Pianin reported from Washington. Staff writer Kathy Sawyer in Washington also contributed to this report.

Retired Adm. Harold W. Gehman Jr., far right, chairman of the board investigating the Columbia disaster, and his panel participate in the board's second formal news conference, at Johnson Space Center in Houston.