SAN LUIS OBISPO, CALIF., DEC. 9 -- Federal investigators recovered a handgun today from the muddy hillside where Pacific Southwest Airlines Flight 1771 crashed Monday and revealed that a third person entered the cockpit before the jet plunged 22,000 feet to Earth.

Shortly before the plane crashed, the pilots radioed that they heard gunshots in the cabin. But most experts said that it was unlikely that the bullets caused the commuter jet to crash unless the pilot and copilot were wounded.

Richard T. Bretzing, special agent in charge of the FBI's Los Angeles office, said a tape from the jet's cockpit voice recorder indicates an "unauthorized entry into the cockpit."

Bretzing declined to describe the sounds picked up on the tape, which was recovered Tuesday. But the recorder has been turned over to the FBI by the National Transportation Safety Board because its content indicates criminal activity, a source at the NTSB said.

Bretzing said that after reviewing the tape, "that suspicion has become extremely strong that the aircraft did become the site of a criminal act."

Patricia Goldman, NTSB vice chairman, said there is no indication that there was anything mechanically wrong with the jet at the time of the emergency.

The investigation into the crash, which killed 43 persons, has been split, with the FBI heading the criminal portion and the NTSB searching for clues to determine why the commuter jet, a BAe146, plowed, apparently nose-first, into the rugged Santa Lucia hills 15 miles northwest of here.

"We have determined from our listening to the cockpit voice recorder there was no problem with the airframe or engines," she said, adding that the flight data recorder -- recovered today and severely crushed -- has yet to be analyzed. The flight data recorder will tell investigators more precisely the instrument and gauge settings and any changes that occurred in flight.

After the crew radioed controllers about the gunfire, the plane disappeared abruptly from radar screens -- an indication that the plane descended steeply, investigators said.

Bretzing declined to describe the gun and said it had been recovered "only an hour-and-a-half" before he began a 12:30 p.m. news briefing.

The FBI investigation is centered on one of the passengers, David Burke, 35, a former USAir Los Angeles ticket agent who was fired " . . . suspicion has become extremely strong that the aircraft did become the site of a criminal act."

-- Richard T. Bretzing

last month, reportedly for pocketing $69 worth of liquor receipts. There have been reports that he might have tried to kill his former boss, Raymond F. Thomson, who also was on the flight bound for San Francisco from Los Angeles.

Thomson lived north of the San Francisco Bay area and commuted to work in Los Angeles. USAir bought Pacific Southwest in May.

Sources have indicated that they believe Burke was carrying a .44-caliber Magnum pistol. Bretzing declined to say whether any bullets had been recovered or whether gunshots could be heard on the recorder. He declined to say how many shots from the gun had been fired.

Goldman said the plane's four engines are among the largest pieces and least damaged of the wreckage. She said no pieces have been located that contain evidence of gunfire, but emphasized that the pieces are very small.

At an evening briefing Goldman said the investigators will begin hand-digging in the crash area tomorrow in an effort to recover more wreckage. They will also use sifting devices, she said.

Witnesses who saw the plane descend said it headed into the hills nose-down and intact. Goldman said the angle of descent has not yet been determined. She said no parts of the cockpit have been found.

At the evening briefing Goldman said that 13 witnesses who saw the plane go down have been interviewed and some of them reported "loud shrieking noises prior to impact." She said that none of the witnesses had any expertise in aviation and that investigators had not discerned what type of noises they described.

San Luis Obispo County Sheriff Edward C. Williams said that by Thursday his deputies will have recovered all the body parts of the victims. None of the victims is intact, the sheriff said. He said most of the wreckage was found within a 200-foot area around a roughly 10-foot-deep crater, gouged out of the hillside by the plane.

The investigation was slowed by severe rain and fog on Tuesday, which added mud to the crash site and obscured smaller pieces of the wreckage.

Air traffic controllers were contacted twice by the pilots before the plane was lost from the radar scope. Before the crash, the pilots sent an emergency signal in secret code to controllers indicating an emergency on board, investigators said. Goldman said the distress signal had been activated "a minute or two" before impact.

The plane carried 38 passengers and five crew members.