Federal safety investigators today removed from a deep crater about a fourth of the wreckage of the executive jet that crashed here Monday after a 1,400-mile flight on autopilot with all six people aboard, including champion golfer Payne Stewart, apparently already dead from oxygen deprivation.

Until now, National Transportation Safety Board workers had confined their search to the perimeter of the impact area, a cow pasture about 15 miles west of Aberdeen in northeastern South Dakota. They had recovered some human remains and parts of the twin-engine Learjet 35 that were spread 150 feet around the crash site in knee-high grass.

Before returning to Washington this morning, NTSB Vice Chairman Robert T. Francis said that because the Learjet was in a vertical dive when it burrowed into wet ground at a high speed, there will be "some slow going" in wreckage so compacted and commingled with mud.

"It's going to be a challenge, with the wreckage and sorting out what's there," Francis said. It will take several days to remove what is left of the aircraft to a hangar at the Aberdeen airport for closer examination, he said. And it could take months more to pinpoint a cause of the accident, which he said he thought was "unique in the history of aviation."

Robert Benzon, NTSB investigator in charge, said tonight at a briefing that searchers found the cockpit voice recorder just before sundown. He said the recorder is not expected to yield any voice data but may have recorded "ambient noises" in the cockpit just before the crash and possibly even frequency sounds that would give clues to the engines' operating status.

"We're very anxious to listen to" the recorder, Benzon said. Earlier, officials had said there would be no recorded voices of the pilots because the device operates on a 30-minute loop and would have wiped the tape clean after the occupants of the plane were presumed to have died of oxygen deprivation.

NTSB workers brought in a heavy-duty backhoe to dig around the 10-foot-deep, 30-foot-wide crater and remove the Learjet's nearly 1,000-pound engines. Francis said that while pieces of the aircraft have been positively identified, including small parts of the cabin pressurization system, there was "not anywhere near enough" evidence to begin making conclusions.

He said some parts of the aircraft's door have been removed from the scene, but there has been no indication yet whether the wreckage yielded any sign of damaged seals, which could have caused a loss of cabin pressure.

Benzon said several oxygen masks were found, but that it was impossible to tell whether they had deployed before the crash. He said that some personal effects, including a golf bag, were also recovered.

Benzon said investigators had determined that a modulator valve that bleeds air from the engines to the cabin pressurization system was changed two days before the crash and worked properly during a short flight on Saturday. He said it was not related to an outflow pressure valve that the Learjet's owner has said was installed at the direction of the Federal Aviation Administration. Benzon said the modulator valve's only interest is that it was a "maintenance anomaly that happened to be repaired just before" the accident.

Initial theories have centered on some sort of explosive decompression--or possibly even a gradual loss of pressure--in the cabin as the aircraft climbed to a planned altitude of 39,000 feet after taking off from Orlando on a flight to Dallas. After losing radio contact with air traffic controllers over Gainesville, Fla., it flew off course for four hours at altitudes up to 45,000 feet, shadowed by Air Force F-16 fighters, before running out of fuel and spiraling to the ground near here.

Although one of the Air Force pilots observed that the Learjet's inside windows had frosted over from the minus-70 degree temperature in the cabin, Francis said that a videotape taken through the windscreen "head-up" display of one of the military aircraft may not be helpful. "The quality is probably not going to be good enough to help us," he said.

CAPTION: Probers lift and remove parts of Learjet that crashed Monday in South Dakota, killing golf star Payne Stewart and five others.

CAPTION: Michael Kling, 43, one of two pilots aboard the aircraft.

CAPTION: Stephanie Bellegarrigue, 27, the other pilot.

CAPTION: Course designer Bruce Borland, 40, a passenger.

CAPTION: Sports agent Robert Fraley, a passenger.