The National Aeronautics and Space Administration announced yesterday that intercom voice recordings from the Challenger crew cabin, restored by IBM engineers, indicate the seven crew members never knew anything was wrong before the space shuttle was engulfed in a ball of fire and broke apart.

"Preliminary analysis of the tape shows the crew was unaware of the events associated with the tragedy, and the internal communications were being maintained as would be expected during a normal ascent," the space agency statement said.

The tape of the astronauts' conversation among themselves breaks off 73.6 seconds after the Jan. 28 launch, the exact time that all air-to-ground flight data and voice communications ceased, agency officials said.

The terse announcement broke NASA's total silence on the emotional subject of the crew's final moments -- one of the few mysteries connected with the disaster not resolved in a report last month by the Rogers Commission.

The agency has not decided whether to release transcripts of the crew conversations, according to NASA spokesman Doug Ward, at Johnson Space Center in Houston, where engineers are listening to the tape.

"Headquarters will make that decision. We've never before released crew transcripts, because they are viewed as note-taking for the crew . . . . But that policy will obviously have to be reviewed in this case because the circumstances are far different." He said the decision would come within a week.

Engineers at Johnson Space Center probably have heard everything there is to hear on the tape, Ward said, "but it's not perfectly clear and some segments require additional review."

The tapes, recovered from the Atlantic more than six weeks after the accident, had been seriously damaged by sea water and caustic chemical byproducts produced by the interaction of the water with the tapes and other material, the agency said. The usual process the agency uses for restoring tapes, such as those recovered from the shuttle's reusable solid rocket boosters, is to wash them in chilled water and then dry them. This procedure did not work in this case, Ward said.

The agency next sent a sample of the tapes to IBM in Tucson, where research has been done on tape restoration. IBM engineers used a mild acid bath to neutralize the damaging chemical reaction and enable the material containing the voice data, which had been separating from its backing, to readhere, Ward said.

IBM returned the tapes to Johnson last week, and NASA engineers began the delicate process of decoding them using special laboratory equipment to convert them from computer-language zeroes and ones into human voices that sound like typical broadcasts from astronauts in space.

Challenger carried five operational recorders containing both voice and computer data. Since the channels are intermixed, the ground equipment is required to disentangle them, Ward said.

In its only other space-connected tragedy, the 1967 Apollo launch pad fire, NASA first announced that the three astronauts had died "instantly" in a "flash fire." However, New York Times reporters found an engineer who had heard a tape of the final minutes inside the spacecraft. The tape showed that the astronauts had fought to get out and called for help over a period of 16 seconds. Three days later, top NASA officials revised their initial version, confirming much of the Times story.

In response to a Washington Post request for information from the tapes under the Freedom of Information Act, NASA responded on May 20, "There are no transcripts of the voice tape recordings recovered from the crew compartment . . . . The data was nonrecoverable."