WASHINGTON - NASA Administrator Sean O'Keefe said yesterday that an independent panel will decide the significance of e-mails by a NASA research engineer warning two days before Columbia broke apart that damage to the shuttle's insulating tiles might have left it in "marginal" condition.

"I'm going to live by that judgment from that independent group to tell us exactly what we could have, should have, might have, would have done had we known something differently," O'Keefe said on CNN's Late Edition.

But O'Keefe insisted the e-mail discussions about the spacecraft were not unusual. "Those are the kinds of dialogues and debates that go on every single time, during every single mission," he said.

"We want to encourage that kind of dialogue and are looking at releasing everything and anything we can find in order to get the maximum evidence and facts together," he said.

The engineer, Robert Daugherty of NASA's Langley research facility in Hampton, Va., wrote that experts on the tiles worried that the shuttle's condition had deteriorated to "survivable but marginal" after it was struck by debris on liftoff.

The critical e-mails were released after news organizations sought them under the federal Freedom of Information Act.

The shuttle came apart Feb. 1 over Texas. All seven astronauts died.

Teams searching for parts shed by Columbia as it broke apart found more small metal fragments yesterday in a rural part of southeast Nevada.

Digital photographs of the material were sent to the Johnson Space Center in Houston for analysis. Several small scraps of aluminum were also found Saturday. NASA has not confirmed whether any debris west of Texas came from the shuttle.

A 10-member independent panel, headed by retired Adm. Harold Gehman Jr., is investigating the accident.

But several members of the panel that investigated the Challenger explosion in 1986 say the board has too many members who are on the government payroll, lacks scientists and doesn't have enough distance from NASA.

The Columbia board was appointed by NASA officials, while the panel that investigated the Challenger accident was appointed by President Ronald Reagan.

"It would put their independence beyond a doubt if they were to report to the president," said David C. Acheson, a member of the Challenger board.

Acheson, 81, a retired attorney in Washington, was one of the 13 members of the Rogers Commission, named after its chairman, the late William P. Rogers, a former secretary of state.

The Rogers Commission was made up of engineers, an astronomer, a lawyer, a space journalist, Nobel Prize-winning physicist Richard P. Feynman, former astronauts Neil Armstrong and Sally Ride, and legendary test pilot Chuck Yeager. Its unsparing 1986 report on the Challenger accident pushed NASA to make significant safety and management changes to the space shuttle program.

Some U.S. lawmakers have criticized the Columbia panel, made up mostly of military leaders and government officials with no household-name recognition who were appointed by O'Keefe a day after the shuttle was destroyed.

In recent days O'Keefe and Gehman have tried to alleviate concerns by expanding the board, hiring an independent staff and changing the board's charter.

"It would be sound policy to have the investigation totally independent of government," said Robert W. Rummel, 87, a retired aerospace engineer in Mesa, Ariz., and Rogers Commission member.

Challenger panel members said they have no doubts about the integrity and the ability of the Gehman board. But they recall NASA's uncooperative approach in the early days of the Challenger investigation and said board members had to push hard for information.

"There was a lot of obfuscation," said Challenger board member Robert B. Hotz, 88, former editor of Aviation Week & Space Technology.

Challenger panelist Albert D. Wheelon, 74, a retired physicist, noted that unlike the Rogers Commission, most of Gehman's board members are government employees.

"If pressure is exerted, can the pressure be resisted, and can the root problem, if it is management, be corrected?" he asked.

The Rogers Commission members said they believe they had an easier task than the Gehman board will have. The Challenger investigators had an immediate leading suspect, the right solid fuel rocket motor; debris that was more easily recoverable along the Florida coast than the Texas terrain; thousands of witnesses; and tracking cameras that allowed them to see what happened in detail.

They also benefited from Congress delaying hearings until their investigation was finished, Acheson said. Congress held its first hearings on Columbia within two weeks of the disaster.