The fault affects the heavy bolts that connect the powerful solid-rocket boosters to the external fuel tanks.
The Columbia Accident Investigation Board, studying the fiery breakup of the Feb. 1 shuttle flight over the southwestern United States, said it worried that parts of these 40-pound, 2-foot-long bolts could break free shortly after liftoff and smash against delicate areas on spacecraft during future missions.
Board officials said the fault involves a "bolt catcher," a container mounted on the fuel tank designed to capture fragments of the attachment bolts immediately after astronauts jettison the powerful booster rockets about 28 miles into their ascent.
Retired Navy Adm. Harold Gehman, the head of the board, said investigators have determined that the bolt catcher was "not as robust as we would want."
Air Force Maj. General John Barry, a board member, indicated the flaw could delay NASA's next shuttle launch.
"This is a possible return-to-flight issue," said Barry, calling the bolts a "pretty heavy piece of machinery."
The 150-foot-tall rockets are mounted on either side of a shuttle's external fuel tank and help provide nearly all the tremendous thrust needed to enter orbit. They are designed to fall away safely into the ocean for later recovery by NASA.
Board members said they do not believe the breakaway bolts — which use explosive devices during the jettison to break the attachment — contributed to the Columbia disaster. But they expressed concern that bolt fragments could cause similarly fatal damage to another shuttle's protective tiles or panels on future missions.
"We're not changing our working scenario," Barry said.
Investigators indicated they remain convinced that a chunk of foam from the external tank smashed against Columbia's left wing, loosening a protective panel along the leading edge. That permitted searing temperatures to penetrate the spacecraft during its fiery return 16 days later, melting key structures aboard Columbia until it tumbled out of control at nearly 13,000 miles per hour. All seven astronauts died.
The latest discovery came during an extensive study of all potential threats to the shuttle, officials said.
The explosive devices on the bolts sever the booster rockets from the external tank within 30 milliseconds of the command to detonate them. The procedure is exceedingly precise — and dangerous — because of risks that one booster might detach before another and send the shuttle tumbling off course or out of control.
Earlier, some of America's top space experts told the Columbia investigators that NASA failed to learn important lessons from its past mistakes and needs to improve its oversight of shuttle contractors.
But they said declining budgets at the space agency may not have contributed directly to the tragedy.
"Spacecraft failures persist, and there is no assurance that lessons are being applied toward future mission successes," said Allen Li, a NASA specialist at the General Accounting Office. GAO has previously criticized NASA for insufficiently managing risks, communicating poorly among employees and using inadequate engineering practices.
Li was among officials testifying at a final public hearing for the investigation board before it retreats behind closed doors to prepare its formal report on the Columbia disaster.
The board has indicated it wants to finish its report prior to the August recess by lawmakers in Washington.
Marcia Smith, who studies America's space program at the Congressional Research Service, reviewed the history of NASA's budget, as requested by the White House and approved by Congress. She cautioned that it will be difficult for investigators to directly tie the decline in shuttle funding to the February tragedy.
"It's very difficult to tie this into events like the Columbia tragedy," she said. She added that it was "not clear that an increased budget would have helped" NASA to appreciate the risks that insulating foam might damage shuttles on takeoff.
The budget for the shuttle approved by lawmakers during the last decade peaked at $4.04 billion in 1993, according to congressional researchers. It fell steadily until it dropped as low as $2.93 billion in 1998 and has gradually risen to $3.27 billion for fiscal 2002.
"It may well be that you'll conclude ... the shuttle budget was cut too far, but it will be interesting to see if you can tie that directly" to events that led to Columbia's destruction, Smith said.
Li told the board that the space agency "is at a critical juncture, and major management improvements are needed." He added that NASA has made "noteworthy progress" in improving its oversight of contract workers, but "much work remains."
The board, after four months of study, has already expressed concerns about the space agency's dwindling budgets over the past decade and NASA's alternating control over shuttles between managers at its Johnson Space Center in Houston and its Washington headquarters.