On an early spring day, an FBI agent on loan to the presidential commission investigating the Challenger accident sat in an office in Brigham City, Utah, leafing through documents concerning the space shuttle's solid-rocket boosters, conducting what he believed to be a routine mopping-up operation.

It seemed unlikely that yet another "smoking gun" could turn up, after more than two months of digging.

But among the papers at booster manufacturer Morton Thiokol Inc. were copies of documents that the National Aeronautics and Space Administration had not provided the commission, despite repeated requests for "everything." The papers showed that last July, six months before the Challenger catastrophe, a NASA official had decided that the booster's O-ring joints were not safe enough for the shuttle to continue flying and imposed a "launch constraint," but that he had then repeatedly overridden his decision, allowing launches to continue.

The agent's 11th-hour discovery, at the end of eight years of ignored warnings and "red flags" about inadequate hardware, prompted the angered commission to schedule a last closed hearing with Marshall Space Flight Center officials responsible for the boosters.

As the hearing convened on May 9, executive director Alton G. Keel recalled last week, panel members still hoped that the Marshall witnesses would "come back and tell us it's not true; tell us we're wrong; tell us we've misread this."

But the testimony confirmed the fears. "It was clear the whole story was there," Keel said, and the commission went public with the latest discovery, releasing the hearing transcript to the news media.

The episode helps illustrate why the commission is likely to be remembered as a model. It cracked the case, will deliver its report on time and achieved this with possibly less secrecy than any predecessor. With its relentless digging, it silenced early critics who thought it would be too sympathetic to NASA.

Chairman William P. Rogers, when asked about the commission's work, has said he was motivated in part by a desire to avoid the controversy and loose ends left by the Warren Commission, which investigated the assassination of President John F. Kennedy.

And the episode shows above all the continuing struggle between the commission and NASA engineers at Marshall, which reflected the commission's growing disillusion with an agency once considered a paragon.

The report of the Rogers Commission, as it has become known, will be released Monday. Because of the panel's openness, it is expected to contain no new shocks.

But when the commission began work, members doubted they would solve the riddle of the Challenger, which disintegrated Jan. 28, killing its crew of seven. In the end, commission members had amazed themselves by finding answers and were even more amazed, and anguished, by what they were.

"We established without any doubt what happened," Rogers said in an interview last week.

The 13 commissioners, experts in their professions -- engineering, industry, law, science, flying -- were brought together abruptly by the White House in February. They began as strangers but, as they became a team, hurrying around the country on red-eye flights and eating fast food over conference tables, they slowly came to know each other.

An Unexpected Side

Neil Armstrong, the panel vice chairman and the first person on the moon, showed colleagues an unexpected side. Although not talkative in public, behind closed doors he was able to galvanize the commission with his blackboard analyses of concepts and battle plans. Rogers and Armstrong were so respectful of each other that it "took a while" for them to get to know each other, a panel member said.

Astronaut and commission member Sally K. Ride, once familiar in NASA public relations tours as the gracious and smiling "first woman in space," emerged early as a grim-faced interrogator of NASA colleagues. For her, the panel's mission was acutely personal. Members of the Challenger crew were friends, and she had flown into orbit twice aboard Challenger.

"All my emotions were still tied up with the accident and its aftermath," Ride said of the day when she got the call from William R. Graham, then acting chief of NASA, asking her to serve on the panel. "I was glad to have something to do."

She was not the only member with an emotional commitment. Albert D. (Bud) Wheelon, a physicist and frequent critic of NASA, took the job partly because, as a top executive with Hughes Aircraft Co., he was the boss of Challenger crew member Gregory B. Jarvis. "We sent him up there. I thought I owed it to his widow" to be on the panel.

And an odd couple emerged: Air Force Maj. Gen. Donald J. Kutyna, the panel's spit-and-polish Pentagon representative, and Richard P. Feynman, a rumpled Nobel Prize-winning theoretical physicist with a penchant for theatrics.

During the first day of public hearings, Kutyna leaned over to Feynman and said, "Pilot to copilot. Comb your hair." It "kind of broke the ice," Kutyna said.

After the hearing, they ran into each other on the sidewalk and were left standing as others drove off in limousines.

Feynman asked Kutyna how he was getting home. Kutyna replied that he was taking the Metro. Feynman grinned and said, "Any general who rides the subway has got to be all right."

Their friendship would play a role in one of the panel's few disputes, which erupted as the final report was being prepared. Rogers wanted to soften the wording in a chapter by Feynman that blistered NASA, and Feynman threatened to withhold his signature from the report. It was Kutyna whom Rogers called on to conciliate.

As for Rogers, the public remembered him, if at all, as a mild-mannered secretary of state under President Richard M. Nixon, overshadowed by Henry A. Kissinger. But Rogers launched the commission at high speed and wielded the skills of his earlier career as a tough lawyer-investigator. Asked about the perception that he had been transformed, Rogers said with a smile last week, "That just shows what short memories people have. I don't think I've changed very much."

Rogers insisted on a number of strategic moves that shaped the commission's course.

*He demanded that the panel's investigation be open, not secret. "He got us to accept the fact that whether we liked it or not, it was going to be a very open operation," a panel member said.

"Full disclosure has advantages over indictments," Rogers said. "You don't want to punish. You just want to make sure it doesn't happen again."

*He averted a political "circus" of hearings and counter-hearings by persuading restive members of Congress, for the most part, to hold off on hearings and investigations.

*He set a daunting example for the other commissioners as a hard worker, putting in long hours. "He was down there all the time. . . . His example motivated the rest of us," one member said.

In the most striking contrast with other commissions, the Rogers panel members, though aided by investigators and staff, did their own work, each bringing an expertise.

Eugene Covert, a professor of aeronautics at Massachusetts Institute of Technology, is a former NASA consultant who was involved in the design of the shuttle's main engines. A metallurgy expert, "he understood the most exotic aspects of the hardware," a fellow panel member said.

Joseph F. Sutter was a principal figure in development of the Boeing 747 jumbo jet. David C. Acheson, a Washington lawyer, was formerly a top executive of the Communications Satellite Corp. Robert B. Hotz is the former editor of Aviation Week & Space Technology. Aerospace engineer Robert W. Rummel was formerly vice president of Trans World Airlines. Charles (Chuck) Yeager, the legendary test pilot, was appointed to the commission but showed up for only one hearing. He told The Washington Post he had previous commitments.

Radically Changed Tone

In fact, the commissioners were forced into their hard labor in part by NASA, and by an incident that radically changed the tone of the investigation.

At an early hearing, on Feb. 11, the commission had appeared protective of NASA when, led by Rogers, members challenged a young budget analyst, Richard C. Cook. He had written an internal memorandum last July warning his NASA bosses that engineers believed that booster seal damage was a potentially "catastrophic" problem, and a reminder memo within days after the accident. A commission source said later that, if members seemed hostile, it was because they were offended by what they at first thought was unseemly haste by Cook to say "I told you so" amidst the grief that followed the accident.

But a few days later, the commission uncovered a fact that began dimming the space agency's halo. The panel had gone to Cape Canaveral for a routine orientation. There, however, the commissioners learned that the night before Challenger lifted off, Morton Thiokol engineers had warned against launch, that their Thiokol bosses had overruled them and that top NASA officials never received news of the debate and the warnings. "I was surprised," Ride recalled last week.

Allan McDonald, the contractor's chief engineer at Cape Canaveral who first revealed that he and other engineers had warned of potential disaster, soon became a hero in the drama, referred to by panel members as a "super good guy."

After the story was laid out in closed hearings on Feb. 14, the shocked panel members sent all witnesses out of the room and caucused for 45 minutes. That session produced rapid-fire decisions to request that NASA bar from its internal investigation those who had participated in the launch decision, that is, those who were investigating themselves; to inform the president immediately, and, perhaps most significantly, to issue a public statement indicating that the investigation was taking a new, troubling turn, that, as Rogers said on Feb. 15, the agency's "decision-making process may have been flawed."

The process changed from a joint inquiry involving NASA and the commission to an investigation steered by the commission alone. "We realized we would have to be a lot more independent," a commission source said. "We realized it would be unnatural to expect the documents we needed to automatically flow to us now."

No one suggested it formally, but many of the panel members increased the time they were spending on the investigation, some virtually abandoning their normal lives.

The commission had just begun to hire a staff, including Keel, a PhD in engineering physics. A former assistant secretary of the Air Force for research, development and logistics, he is detached from the Office of Management and Budget where he is director for national security and international affairs.

The full-time staff eventually grew to 43, including 15 investigators, plus about 140 part-time contract specialists.

The commission interviewed more than 160 people and held more than 35 formal investigative sessions generating more than 12,000 pages of transcripts. They gathered more than 6,300 documents, totaling more than 122,000 pages, and examined hundreds of photographs.

Most of the panel members, as well as staff investigators, fanned out around the country in teams to NASA centers and contractor plants.

Arthur B.C. Walker Jr., a physics professor at Stanford University and aerospace consultant, would teach a class by day and then hop a red-eye night flight to do his commission sleuthing around the country. With his team, he made 17 trips in two months, some transcontinental, to NASA centers in Houston, Cape Canaveral and Huntsville, Ala., and contractor plants in California, Utah, Louisiana and Kentucky, as well as attending commission meetings and hearings in Washington.

The technical cause of the accident was determined by a commission subgroup under Kutyna, the only member who had experience investigating a rocket accident. He had led an investigation of a Titan missile explosion some years earlier and drew up the plan the panel followed during most of its 120 days.

Tracing the Cause

His group traced the cause to a failure of the O-rings to seal a joint between segments of the right solid rocket booster. The segments are essentially tubes stacked so that the lower edge of one tube fits into a groove cut into the wider top edge of the the tube below. In cross-section this looks like a tongue-and-groove joint.

The gap between tongue and groove is supposed to be sealed by two synthetic rubber O-rings. Each O-ring fits loosely into its channel in one inner wall of the groove so that it can press against the tongue of the upper segment to seal the gap.

As soon as the booster ignites, the pressure of hot gases is supposed to press the first O-ring, making it bulge into the gap. If the O-ring does this, a step called actuation, the pressure can only tighten the seal. If the O-ring fails to actuate, however, the gases can slip past, grazing the O-ring with enough heat to burn the rubber.

"Understanding pressure actuation is the key to understanding what happened, but it's now clear NASA didn't understand," a source close to the investigation said. "They just flat didn't understand how these joints work."

Because so much of the commission's work was out in the open, the public shared in many of the crucial discoveries as they emerged.

Even before the first public hearing, one important piece of evidence had been released by NASA: a film showing an "unusual plume" erupting from Challenger's right-hand solid rocket booster about 15 seconds before the fireball erupted. NASA refused to say what the plume might have been, but already it seemed inescapable that the booster had ruptured and was spewing white-hot flame. Still, it was unclear what caused the plume and how it led to the final conflagration.

But at the commission's first open hearing, on Feb. 6, commission members began zeroing in on the one factor that distinguished the launch from all others: the record cold.

And at the Feb. 11 hearing, Feynman conducted an impromptu experiment on the hearing-room dais. He squeezed a piece of O-ring in a C-clamp and dunked it in his ice water for a minute. When he removed the O-ring and took off the clamp, the ring maintained a flat spot where the clamp was, taking several seconds to rebound.

"I believe," said Feynman triumphantly, "that has some significance for our problem."

Some panel members saw Feynman's experiment as an attempt at "showboating" and at first wondered if this tendency might get in the way of his work. They concluded that it did not, much. "He doesn't really like working with a group," a fellow panel member said. "He's a loner, a very reflective guy. He's brilliant, sees relationships that other people don't see. He got very impatient at the early public hearings. He was anxious to go into the field and get his hands on metal."

By opening the clamp on a cold O-ring, Feynman simulated the widening of the joint gap in the first second of Challenger's flight that frosty morning of Jan. 28, when four-foot icicles draped the launch pad.

Together, the commission and the public learned that NASA officials saw the first evidence of O-ring failure within hours after the accident, film they would withhold for more than two weeks showing smoke billowing from a booster joint in the first second after ignition.

Conclusion of Commission

The commission has concluded that the smoke was produced by the burning of O-ring rubber, grease used to slide the O-rings into place during booster assembly and putty that insulates the O-rings from burning fuel. NASA officials told the commission they did not consider the film significant at first.

Ultimately, the commission hired its panel of independent technical experts and had them supervise more than 300 experiments, most on subscale models of the booster joints, at Marshall and Morton Thiokol. Many of the tests simulated conditions of the Challenger launch and reproduced key elements of the catastrophe.

NASA's first error, they learned from engineers at Parker Seals Corp., which makes the O-ring rubber, was to design a joint that used O-rings but is not rigid.

Parker officials told commissioners that the rings were never intended to be used in joints that flex. When the booster fires up, the pressure of burning rocket fuel bulges the booster walls outward, causing the joints to flex in a way that pulls the O-rings away from the gap they must seal.

"It turns out that when NASA first designed this joint," a commission source said, "they got it exactly wrong. They thought the pressure would tend to close the gap, to squeeze down on the O-rings. It turned out just the opposite. The gap opens."

The cold weather on launch day stiffened the O-rings so that they could not bulge into the gap. The temperature had been in the low 20s the night before and had warmed to only 38 degrees at launch time. The cold also kept the O-rings from rebounding from their squeezed shape to maintain a seal as the gap opened.

Experiments conducted for the Rogers commission found that if the joint is cooled below 50 degrees it loses resiliency and cannot rebound enough before hot gases slip past, eroding the rubber.

Yet another factor discovered in the experiments, and apparently unknown to NASA and Morton Thiokol, was the effect of too much squeeze on O-rings. If either of the two segments that meet at a joint is slightly elliptical, the gap will be too large in some places and too small in others.

The tests showed that if the squeeze is too tight, the O-ring's rubber flattens severely against the channel walls. As a result, there is not enough available surface on one side of the O-ring for the pressure to push against to actuate the O-ring. Parker officials told the commission that the company recommended putting a spacer in the channel to keep open the space that the pressure must fill to actuate the O-ring. NASA designed no such spacers for O-ring channels.

Answer From Experiments

One of the mysteries posed by the early puffs of smoke was why the leak seemed to seal itself, at least temporarily. Again, experiments gave an answer.

When scale model boosters were fired with deliberately damaged O-rings, they leaked smoke, then sealed themselves and finally leaked again. When technicians opened the test joints, they found the leaks clogged by an accumulation of brittle, glassy combustion products.

The commission has concluded that when Challenger's booster started puffing, some of the combustion products stuck to the walls of the joint and eventually sealed it. Then, at about 40 seconds into the flight, Challenger encountered a series of severe wind shears, the worst ever encountered by a manned rocket. As the shuttle flew upward, it passed through many layers of wind blowing in opposite directions. For 22 seconds the shuttle bounced back and forth sideways as it climbed.

At about the same time Challenger was passing through a normal period of high aerodynamic turbulence that vibrated the craft. "It was a very bumpy road," a commission source said, "and we think this is what broke loose those glassy combustion products."

At 58 seconds into the flight and with the O-rings already badly damaged, if not destroyed, the booster's gases flared out in a white-hot plume that melted a steadily growing hole in the booster wall and led to the final white cloud that was initially interpreted as an exploding fireball.

In fact, Feynman determined, the mixture of hydrogen and oxygen did not so much explode with a bang as burn rapidly with a whoosh.

In analyzing the human failings that contributed to the accident, panel members were particularly disturbed by what one called the "technical weakness" of some of the engineer-managers responsible for development of the booster joints at NASA and Morton Thiokol.

The most glaring problem, in the view of most, was the "holes" in the management system. Washington headquarters had relinquished too much authority to the centers. And there was an elaborate "pagoda" of paper, as one commissioner described it, designed to flag problems, but much of it was either ignored or never reached the right people.

Among the techniques the probers used to penetrate the thicket of NASA technical jargon was "playing dumb," investigator William Dupree said. After pressing doggedly for an explanation of some phrase, he said, they often found that the NASA experts "didn't really understand what they were saying themselves. I don't think they understand those 'launch constraints.' I don't know anybody who does."