The large pieces of the shuttle Challenger recovered from the sea show surprisingly few signs of scorching or intense heat from the explosion that destroyed the spacecraft and killed its crew of seven in Tuesday's disaster, NASA officials said today.

Officials said that about 3 tons of shuttle debris had been recovered by late today, including a large fragment of the fuselage bearing the haunting word "Rescue," in black letters on a yellow background, that had pointed to an escape hatch. The fragment has been identified as coming from the lower right side of the middeck crew compartment.

Asked about what preliminary clues might be seen from the fuselage parts, a spokesman for the National Aeronautics and Space Administration said he was surprised at the apparent "lack of burning" on the fragments. "I expected to see charring," said Jim Mizell, a retired engineer at the space agency.

"There is much more chance to recast whether the orbiter survived the blast or fell into the water intact," he said. "This makes the engineer's job and the metallurgist's job a little easier."

At a briefing tonight, the space agency said two unmanned, camera-laden submarines are in the vicinity of where the largest fuselage pieces have been recovered, in the northeast portion of the 8,000 square mile search area. The submersibles are seeking to locate and identify a large object that was detected by sonar sweeps. A strong current kept divers from investigating the site today.

Responding to reporters' speculation that the object might be some large part of Challenger's crew compartment, Coast Guard Lt. Cmdr. James Simpson said: "It could be a Spanish galleon, or a shrimp boat; it could be a bunch of rocks, or it could have something to do with Challenger. There's no way of telling . . . . "

Although the pressurized crew cabin is solidly reinforced, some engineers believe it might have burst and that the astronauts' remains will never be found.

Others knowledgeable with space flight have repeatedly expressed the belief that the crew could not have survived, even if Challenger had been equipped with ejection seats, as the first shuttle initially was.

In Houston, the seven-man investigation board met today at the Johnson Space Center to review preliminary reports of the wreckage retrieved in the ocean over the last three days off Cape Canaveral.

At the same time, support groups from Johnson Space Center in Houston, the Kennedy Space Center in Florida and the Marshall Space Flight Center in Huntsville, Ala., began the task of pulling together the almost 10 million bits of information radioed to Earth by Challenger in the 74 seconds of its tragic flight.

"The process is progressing well but is still in its early stages," said Jesse W. Moore, NASA's associate administrator for space flight and chairman of the investigative board.

The board plans to return to Florida Saturday and hold its next meeting on Sunday, when its first report will be made public.

Meanwhile, the chairman of the House aviation subcommittee suggested that an independent panel of experts who are not employed by the space agency be appointed to assist in the investigation.

Rep. Norman Y. Mineta (D-Calif.) said an independent board would be "the most credible" way to study the accident. "The public has come to expect the accountability and integrity that an outside panel provides," he said.

In Florida, NASA spokesman Mizell said the retrieval of some large pieces from Challenger increases the chance that the disaster review boards may be able to determine the cause of the tragedy.

The large fragment bearing the word "Rescue" also shows an arrow that pointed to a small escape hatch in the side of the ship.

Astronaut Gregory B. Jarvis, a payload specialist, was sitting within a few feet of that hatch. Christa McAuliffe, the teacher-astronaut whose death so added to the nation's stunned sense of loss, was sitting slightly forward of Jarvis and to his left. Ellison S. Onizuka was sitting on the left side of the compartment. The middeck is beneath the flight deck, where Challenger's commander, Francis R. (Dick) Scobee, and the three other astronauts were stationed.

The "Rescue" fragment, measuring about 25 feet in length and 6 feet high, is among the largest pieces of Challenger pulled from the Atlantic Ocean thus far. A flotilla of Coast Guard and Navy ships, and about a dozen aircraft, continue searching for debris.

NASA spokesman Mizell has not seen the fragments recovered thus far. His remarks were based on his viewing of a videotape taken by a NASA camera crew when the Coast Guard cuttter Dallas returned here early this morning from the search area and unloaded the debris for storage and analysis by investigators. Mizell said structural and chemical analysis may determine whether the orbiter was destroyed by the initial blast or whether it remained somewhat intact, only to be demolished when it struck the water after falling more than nine miles.

Searchers were concentrating in the northern part of the search area, about 50 miles northeast of Launch Pad 39-B, where the ill-fated shuttle took off Tuesday at 11:38 a.m. Coast Guard Cdr. Robert Bender, a helicopter pilot who has been part of the aerial search effort this week, said the pieces of debris "seem to be getting smaller all the time."

The fragments are being stored and inspected at a hangar-like building at the Kennedy Space Center here. Special precautions are being taken to guard against possible injury from the extremely toxic fuels used in the Challenger. It is unknown if any shuttle portions containing fuel tanks or other dangerous chemicals have been recovered.

Working with an air of grim determination, the search fleet combed the ocean at half-mile intervals, executing tight turns, and dropping smoke bombs when large pieces of debris were found.

But time, wind, and tide are working in their own way in the Atlantic off the spaceport. A pool reporter, Jim Slade of Mutual Radio, after touring the search zone with the Coast Guard, said the debris is being dispersed across a wider and wider area of the ocean. Soon, he said, it will be so widely scattered, so nondescript amid the waves and tides, that "it would be meaningless to a passerby unaware of the tragedy."

Washington Post correspondent Thomas O'Toole reported from Houston that two former astronauts who attended the memorial service today at the Johnson Space Center said they think that the explosion that destroyed the spacecraft began either in the huge tank that holds 1.6 million pounds of liquid hydrogen and oxygen or in the three main engines that burn these fuels.

The two men, who asked that they not be identified, ruled out any direct involvement in the explosion of the two towering solid-fuel rockets, which veered out of control after the explosion and were destroyed by the Air Force range safety officer to prevent one of them from striking the beach.

The two former astronauts, who have watched the slow-motion films of the explosion hundreds of times, said they are convinced that the blast was triggered by a leak or rupture of the shuttle's fuel tank or an explosion in one of the engine chambers where the liquid hydrogen fuel is burned.

"That's the way it looked to us no matter how many times we've looked at that film," one of the men said. "If you follow those two solid rockets, you'll see they're still intact enough to try to fly on their own even though they're aimlessly moving around the sky. Those boosters still had their guidance systems working, trying to lift them up into the sky with the fuel they still had left."

The three engines that burn the liquid hydrogen in the shuttle operate near the edge of rocket technology. The high-speed pumps that deliver hydrogen fuel and oxygen to each engine's combustion chamber are rated at 77,000 horsepower.

The three engines that took Challenger off the launch pad Tuesday had seen considerable duty. One engine had flown four times; the other two had flown five times each. The three engines, including actual flight time, had been started up and run 29 times, for a total of 2 hours, 42 minutes and 25 seconds.

A reporter's attempt to obtain information about maintenance on the three engines was turned down by the public affairs office at the Kennedy Space Center. A space center spokesman said maintenance information on Challenger's three engines had been impounded by the investigative board. The reporter made a second attempt for the information by calling Rockwell International, which makes liquid fuel engines for the shuttle. The reporter was told that NASA had ordered the company not to release information about the engines.

The former astronauts said they could not understand the ball of flame that crawled up the side of the hydrogen fuel tank during Challenger's flight and then disappeared in the explosion. They said they have looked at that part of the film more than any other segment.