Fire had broken out in the passenger cabin of doomed ValuJet Flight 592 and passengers were apparently having trouble breathing just six minutes after takeoff from Miami, investigators said yesterday following a preliminary reading of the DC-9's cockpit voice recorder.

The recorder, recovered Sunday after a 15-day search of the Everglades, added to growing evidence that a roaring oxygen-fed fire broke out of a cargo hold into the passenger cabin, bringing the plane down within minutes and killing 110 people.

Robert Francis, vice chairman of the National Transportation Safety Board, said the voice recorder was badly battered but survived the May 11 plunge into the Everglades. He said there are indications that there might be some gaps in the tape, and that transcribing it and deciphering the various sounds will be difficult.

"This is not going to be a question of hours, but of days," he said in a briefing less than 24 hours after a Metro-Dade homicide detective's probe hit the much sought-after recorder's protective shell in a few feet of water and muck.

However, Francis said, a preliminary reading revealed a normal flight followed by a rapidly deteriorating situation.

"Approximately six minutes of data appear where the aircraft was apparently in a routine situation," Francis said. "That is to say, there's no indication that there's a problem in the first six minutes.

"It appears that the cockpit door opened -- and I say appears -- but there were verbal indications from the cockpit that there was fire in the passenger cabin. There was also an indication from the cabin that there were problems obtaining oxygen."

The plane crashed about four minutes after the first report of fire. According to earlier information from air traffic control radio transmissions and the plane's flight data recorder, a total of 10 minutes and 16 seconds elapsed from the time the plane was cleared to take off to the last, inaudible radio transmission from a crew member as the nearly full twin-jet neared the end of its final 7,000-foot plunge.

Francis would not elaborate on whether "obtaining oxygen" meant that air in the cabin was becoming hard to breathe or that flight attendants were having trouble pulling oxygen masks from the ceiling in an attempt to help passengers breathe. He said that obtaining answers to such questions will be "an enormously sophisticated and complicated and time-consuming task."

However, aviation experts pointed out that oxygen masks are designed to provide supplementary oxygen in cabin decompressions, and are useless in smoke. The masks blend oxygen with available cabin air, and if that air is contaminated, the passengers will merely get oxygen-rich contaminated air.

Investigators also will ask why a flight attendant opened the cockpit door to report the problem rather than follow established procedure to call the pilots by intercom.

While stressing that the information is preliminary, Francis said in answer to a question that "there was some communication, which was not by intercom, from the cabin to the cockpit."

Bernard Loeb, director of the board's office of air safety, said the tape will be difficult to decipher because there is heavy cockpit background noise, and possibly other problems caused by the 15-day immersion in the murky waters of the Everglades. But he said that the magnetic tape is "generally in good condition" and that the board's laboratory equipment should be able to electronically filter out the background noise.

The cockpit voice recorder can provide a wealth of information beyond what the crew said. The board's sophisticated audio equipment can spot and often identify unusual sounds, and can usually determine how far away the sounds were and the direction from which they came.

One of the major questions the board will want to answer is whether the crew donned oxygen equipment and special smoke goggles. Investigators who have listened to air traffic control radio tapes are divided over whether the copilot's voice is muffled in a way that would be typical of someone wearing a mask.

So far, investigators know that some of 119 hazardous oxygen generators being carried as cargo were involved in a fire in the forward cargo hold, along with several inflated aircraft tires that could have produced thick black smoke when burned.

The oxygen generators normally are used in the aircraft cabins of some planes to produce oxygen for passenger masks. They contain a "candle" of sodium chlorate and iron powder which, when burned at high temperature, produces oxygen. Such canisters are not used in DC-9s.

When the canisters are installed above passenger seats, they are well-insulated because they produce external heat of up to 500 degrees Fahrenheit.

The generators, about the size of a can of tennis balls, had been returned to ValuJet by its Miami contract repair facility, SabreTech, because their useful life had expired. Apparently because of a mix-up at the repair facility, they were marked "empty."

ValuJet has never asked for permission to haul such hazardous cargo, but a ValuJet employee loaded the supposedly empty generator canisters loosely into the cargo hold, sitting at least one cardboard box on top of a tire. That would place it in a position where it could turn over on takeoff. The canisters had no protective plastic safety caps, meaning a relatively slight jostling might set them off.

What is not known is whether the canisters initiated the fire or whether the fire started somewhere else and migrated to the cargo hold. Investigators are looking hard for the still-missing main circuit-breaker board from behind the captain's seat, which a mechanic worked on before the plane took off that same day on an earlier flight from Atlanta to Miami.

Also not known is what caused the final loss of control that began a 40-second, 7,000-foot dive into the Everglades. Possibilities include that the crew was incapacitated or that control cables running beneath the floor were burned through. Aviation experts said that in such a hot cargo-hold fire, it would not be necessary for the cables to burn through because they could be jammed by fire-softened floor beams that can sag under passenger weight. CAPTION: Bernard Loeb, director of aviation safety for the National Transportation Safety Board, displays cockpit voice recorder from ValuJet Flight 592 while NTSB Vice Chairman Robert Francis takes a question at news conference.