"There's gunfire on board . . . We're going down."

The distress call came from Pacific Southwest Airlines Flight 1771 halfway on its run from Los Angeles to San Francisco, flying at 22,000 ft. Two minutes later, the British Aerospace commuter jet shrieked toward earth in a nearly vertical dive and disintegrated as it slammed into a hill near Paso Robles in San Luis Obispo County. All 43 aboard were killed, including four executives of Chevron Corp. From that baffling beginning, other messages gradually unraveled the mystery of what had happened.

"Jackie, this is David. I'm on my way to San Francisco, Flight 1771. I love you. I really wish I could say more, but I do love you."

The message on the answering machine of USAir Ticket Agent Jacqueline Camacho in Los Angeles was from her estranged boyfriend. David Burke, 35, was also a USAir agent, who had been fired on Nov. 19 after he was caught stealing $69 from flight cocktail receipts by a hidden camera. Born in Britain of Jamaican parents, Burke had never married but had fathered seven children by four women. After his dismissal, he turned moody and violent. He had held Camacho and her six-year-old daughter at gunpoint on a forced six-hour auto drive the previous Friday, and he seemed particularly bitter toward the boss who fired him: Raymond Thomson, 48, the USAir customer-service manager in Los Angeles. Thomson commuted regularly by air from his Tiburon home in San Francisco Bay.

"David Burke had been allowed to bypass security screening as a familiar airlines employee."

That was how an FBI affidavit described Burke's boarding of Flight 1771 after purchasing a one-way ticket. Thomson, heading home, got on the same plane.

"We've got a problem here," said a voice on the cockpit tape recorder recovered at the crash site. Then came other, more ominous sounds.

The tape had recorded gunshots, then the sound of pounding on the cockpit door and what the FBI termed the "unauthorized entry" into the flight deck. This was followed by scuffling and shouts in what one investigator described as "a terrible commotion." Finally came a high whine, presumably created by air rushing out of the pressurized cockpit through a bullet hole in a window or wall. Patricia Goldman, head of the National Transportation Safety Board's on-site investigators, said they could find "no apparent problems with the aircraft, frame, structure or engines" that would have led to the crash. Other investigators suggested that both the pilot and copilot had probably been shot. An inert body, slumped against the controls, could throw the plane into a dive.

"There is evidence to believe that David Burke was involved in the destruction of PSA Flight 1771."

That statement from the FBI affidavit was based on evidence found by the probers who picked over the muddy hillside. The grisly discoveries: one of Burke's thumbs, identified by its print, proving he had boarded the flight, and a Smith and Wesson .44 magnum revolver with six empty casings. The FBI found a USAir employee who said Burke had borrowed the gun from him last month. Most incriminating was a note, written in Burke's hand, on the outside of an air-sickness bag. It read:

"Hi, Ray. I think it's sort of ironical that we end up like this. I asked for some leniency for my family, remember. Well I got none. And you'll get none."