The space agency revealed yesterday that, an instant before a spreading fireball ended all communications aboard the doomed shuttle Challenger, pilot Michael J. Smith uttered one last phrase: "Uh-oh." For several more seconds, at least some crew members apparently remained conscious and tried to activate emergency oxygen supplies.

The crew may have remained conscious for six to 15 seconds or longer as the ship began to break up at 48,000 feet. The space shuttle then soared to its peak altitude of 65,000 feet before plummeting in 165 seconds to a certainly fatal impact in the Atlantic Ocean at a speed of 207 miles per hour.

The seven astronauts may even have been alive, conscious or unconscious, when Challenger hit the water, the National Aeronautics and Space Administration said.

These new details, the first evidence that the crew knew what was happening, were released by NASA yesterday in its official report on how the astronauts died. The information was based on an analysis of tapes recovered from the wreckage and restored by technicians, and on an analysis of the wreckage itself.

Yesterday's report was at odds with a NASA announcement on July 17, which said that preliminary examination of partially restored intercom tape from Challenger indicated that the crew "was unaware of the events associated with the tragedy . . . . " Yesterday, officials said that more detailed analysis revealed the new findings.

The exact causes of death of the seven crew members could not be positively determined, the report said. "The impact of the crew compartment with the ocean surface was so violent that evidence of damage occurring in the seconds which followed the explosion was masked," Dr. Joseph P. Kerwin, the former astronaut who headed the investigating team, said in his report to Richard H. Truly, head of the shuttle program.

The findings, presented by the two men at a news conference at NASA headquarters here, came exactly six months after Challenger disintegrated, grounding the shuttle fleet for at least two years and prompting an overhaul of NASA management.

The astronauts' moments of "useful consciousness might have varied between six and 15 seconds," Kerwin said, depending on how rapidly the crew compartment lost pressure.

Kerwin's team also found that "the forces to which the crew were exposed during orbiter breakup were probably not sufficient to cause death or serious injury," and that "the crew possibly, but not certainly, lost consciousness in the seconds following orbiter breakup due to in-flight loss of crew module pressure."

NASA released a transcript of the final moments of conversation in the crew compartment -- the first time the space agency has made public such tapes of astronauts' private conversations. The tape, restored by IBM engineers after its recovery from the bottom of the Atlantic, begins with normal exchanges, including laughter and exuberant banter.

"There they go, guys," said shuttle commander Francis R. (Dick) Scobee when Challenger's three liquid-fueled main engines fired up six seconds before liftoff.

"Allll riiight!" responded mission specialist Judith A. Resnick.

"Go you mother!" pilot Smith said 11 seconds after launch, as Challenger rolled into its proper flight trajectory.

Nineteen seconds into the flight, Smith noticed the record wind shear forces buffeting the vehicle, which were later cited as an added stress factor by accident investigators: "Looks like we've got a lotta wind here today."

Sixty seconds after launch, Smith said, "Feel that mother go," and an unidentified crew member responded, "Wooooohoooooo!"

Scobee spoke the last words heard over Challenger's ground to air communications linkup -- "Roger, go at throttle up." -- 70 seconds after launch, and three seconds later Smith said "Uh-oh."

While initial television images suggested that the entire vehicle had exploded, videotape viewed later showed the crew compartment shooting away from the craft and spiraling down to the sea by itself.

Teacher-in-space Christa McAuliffe, her fellow payload specialist Gregory B. Jarvis and mission specialist Ronald E. McNair were seated in the middeck, one level below the flight deck where the others were. As is routine, the two presumably were monitoring the crew conversations but did not speak.

NASA notified the families of the astronauts of the contents of the tape and findings of Kerwin's team before making them public, Truly said.

Challenger broke apart 73 seconds after launch on Jan. 28 when a leak in its right solid rocket booster triggered the eruption of fiery gases from the shuttle's external fuel tank. The craft was traveling about twice the speed of sound. The fate of the astronauts, as indicated by recovered wreckage and photographs of the disaster, depends on what forces they experienced as their compartment tore away and how rapidly it decompressed in the thin air at that altitude, Kerwin said.

"The forces on the orbiter at breakup were probably too low to cause death or serious injury to the crew but were sufficient to separate the crew compartment from the forward fuselage, cargo bay, nose cone, and forward reaction control compartment," the report said.

"The largest acceleration pulse occurred as the orbiter forward fuselage separated and was rapidly pushed away from the external tank," the report continued. "It then pitched nose-down and was decelerated rapidly by aerodynamic forces. There are uncertainties in our analysis; the actual breakup is not visible on photographs because the orbiter was hidden by the gaseous cloud surrounding the external tank."

The maximum forces on the Challenger probably produced from 12 to 20 times the pull of gravity for "quite brief" periods, the report said, and "these accelerations are survivable . . . . "

The forces on impact with the sea were estimated to be about 200 Gs -- far in excess of survivable levels.

Evidence that at least some of the crew members tried to save themselves came from an analysis of the wreckage recovered from the sea. When the crew compartment separated from the rest of the orbiter, the crew lost the normal oxygen supply except for a few seconds' worth left in the lines.

But each member's helmet was also connected to a "personal egress air pack (PEAP)," which contained an emergency supply of breathing air -- not oxygen -- intended for use in ground emergencies.

Four of these packs were recovered, Kerwin said, and three appeared to have been activated manually by the crew members before impact with the water, according to the report.

The PEAP not activated was Scobee's. One of those activated was Smith's. The others could not be connected with any particular crew member.

Since the switch that activates the PEAP is hard for a crew member to reach without taking off his harness, said Truly, a former astronaut, "I would imagine that either El Ellison S. Onizuka or Judy Resnik . . . turned on Mike's Smith's air pack."

"PEAP activation could have been an instinctive response to unexpected loss of cabin pressure," the report said. However, the air in the packs would not have prevented rapid loss of consciousness after they were activated.

Crew seats were apparently in place and occupied at water impact and all safety harnesses locked, the report said.

Kerwin, now director of life sciences at the Johnson Space Center in Houston, said the team spent much of its effort unsuccessfully trying to determine whether the crew cabin lost pressure before water impact.

Until the July 17 announcement that the tapes had been restored, NASA officials had maintained an official silence concerning the manner in which the astronauts died, emphasizing the sensitivity of the subject and expressing concern for the bereaved families.

In response to a Washington Post request for information from the crew intercom tapes under the Freedom of Information Act, NASA responded on May 20 that the data was "nonrecoverable." NASA officials later said they had believed that to be true at the time, adding that the restoration by IBM engineers was a "minor miracle.