SAN LUIS OBISPO, CALIF., DEC. 8 -- The crash of a commuter jetliner Monday that killed 43 people may have been caused by a disgruntled former airline ticket agent who boarded the flight with a pistol to kill the man who fired him, FBI officials said today.

Richard T. Bretzing, special agent in charge of the Federal Bureau of Investigation's Los Angeles office, said after viewing the wreckage of Pacific Southwest Airlines (PSA) Flight 1771 scattered over oak-studded hills 15 miles northwest of here that "at this point it does not appear that it was an accident."

He said a force of more than 30 agents, one of the largest FBI contingents to investigate an air crash, is focusing on the possibility "that it was a criminal act on board that caused the craft to come down."

By late afternoon, investigators from the National Transportation Safety Board had inspected the scene, in what NTSB vice chairman Patricia Goldman described at a briefing tonight as a joint effort with the FBI.

"Even though there may be more solid evidence of criminal activity coming out, that still does not tell us why the plane crashed," she told reporters. She added that to her knowledge a plane has never been brought down in this country by gunshots.

She said investigators, who were hampered by heavy rain, have not found a gun or any piece of the wreckage that shows evidence of being hit by a bullet. But she cautioned that the investigation is in its earliest stages and not all the wreckage -- scattered over a wide area -- has been examined.

Bretzing indicated that the investigation has targeted David A. Burke, 35, who was dismissed Nov. 19 as a USAir customer service agent at Los Angeles International Airport for misappropriation of funds. Burke had worked 15 years for USAir, which purchased PSA for $400 million in May.

A spokeswoman for USAir said Burke and his former supervisor, customer service manager Raymond F. Thomson, 48, were among the 38 passengers and five crew members who died in the crash, which occurred at 4:17 p.m. Monday.

An FBI source said agents were checking reports that Burke learned that Thomson would be on the flight and used his airline identification to bypass a security checkpoint, enabling him to take a concealed .44-caliber Magnum and six rounds of ammunition on the plane. The source said he could not confirm an ABC News report that Burke had left a suicide note or tape indicating his plans. A USAir spokeswoman declined comment on the report.

Margery Craig, a PSA spokeswoman at the crash site, said that although all ordinary passengers are checked for weapons, it is possible that someone with airline identification would be allowed to bypass the airport security checkpoint. A USAir spokeswoman, however, said Burke had surrendered his identification card when he was fired and that it had been destroyed.

The FBI investigation, which included about 15 agents in blue jumpsuits combing the widely scattered wreckage, began almost immediately after the crash of the British-made BAe 146-200. Shortly before the plane disappeared from radar screens during a flight from Los Angeles to San Francisco, the pilot issued a distress call and told an air traffic controller "there's gunfire aboard." He spoke of smoke in the cockpit and later said, "we're going in," according to investigators.

Heavy bullets fired at close range could cause serious damage to a civil aircraft such as the BAe146, "depending on where they hit," said one aviation expert familiar with the airplane who asked not to be identified.

An explosion could occur if the bullets struck the oxygen tanks that supply the onboard emergency masks, but the tanks are located deep below the passenger cabin on the BAe146 so that ground crews can reach them easily, the expert said.

Airplanes are designed so that the most sensitive systems, such as hydraulic lines and electrical wires, are duplicated; hitting one should not disable the aircraft. Puncturing the cabin walls would cause some loss of air pressure, but that alone would not cause the airplane to crash, he said.

If the pilots were shot, that also could have caused the crash, he said.

Another source said that if a bullet punctured a fuel line or started a fire, it might cause the plane to crash. But other experts said it is unlikely that fuel tanks would explode even if hit directly by a bullet.

Jim Hartzell, whose family runs the 2,000-acre ranch where the plane hit, said he heard an enormous explosion and then saw smoke pouring up into the cloudy sky behind a hill. He said a family friend saw the aircraft coming in low at a steep angle and told Hartzell's sister, Mary Wiley.

"Those Navy pilots often fly low," she replied.

"Not this one," he said. "It's going into the hill."

San Luis Obispo County Sheriff Edward C. Williams emerged from the crash site, sealed off by investigators, to describe "a very vast devastation, with all kinds of things, body parts. It is a terrible situation."

Hartzell, out on his horse this morning looking for cattle that might have been hurt by debris, said he rushed to the scene but left quickly when he saw there was nothing he could do. He said the aircraft appeared to hit intact, digging a 12- to 14-foot hole on the side of the hill, and then to have exploded, scattering debris over at least 20 acres.

PSA identified the pilot as Capt. Gregg N. Lindamood, 43, who had 14 years with the airline including 1,500 hours flying the quiet British commuter jet.

Also killed in the crash were James R. Sylla, president of Chevron USA, three other Chevron executives and West German space scientist Wolfgang Studemann. Other German scientists were traveling with Studemann to a conference in San Francisco, but PSA indicated that they could not be immediately identified because of difficulty in notifying their families.

Early this afternoon FBI agents were interviewing Burke's daughter and girl friend at his town house in a new complex in the port city of Long Beach, 25 miles south of Los Angeles, according to neighbors and FBI officials at the scene. Police said they expect a search warrant will be issued.

The Federal Aviation Administration since 1973 has required commercial airlines to screen almost all passengers and carry-on baggage for weapons or explosives. The airlines hire private firms to conduct the screening with X-ray machines, metal detectors and personal searches. Each method is subject to mechanical or human error.

Airport screeners failed to detect one in five imitation bombs and weapons planted in carry-on luggage by FAA undercover investigators in tests conducted last year at 28 major airports, the General Accounting Office reported in June.

The FAA said the 80 percent detection rate was sufficient to deter would-be hijackers, but it conceded that the rate varied from 99 percent at one airport to 34 percent at another. The agency would not disclose individual airport performance, but one source familiar with the report said yesterday that Los Angeles International ranked "among the five or six worst, well below average."

The FAA, under pressure from angry members of Congress, tightened its enforcement policies as of Oct. 1. A report on the effectiveness of the new methods is to be compiled next year.

Since 1973, more than 38,000 firearms have been detected by screening more than 8 billion passengers, said FAA spokeswoman JoAnn Sloane. She said there have been 16,000 arrests.

In the last 10 years, there have been 92 attempted or actual hijackings, of which 14 involved a firearm, she said. In four of those cases, the weapons escaped detection in the passenger screening process; in eight cases the individuals boarded the planes without passing through the official checkpoints; one involved a commuter aircraft for which screening is not required; one involved a weapon hidden in the airplane lavatory, Sloane said.

If Monday's crash is eventually blamed on gunfire, it will be only the second such crash in California history. The May 7, 1964, crash of a Pacific Airlines twin engine turboprop F27, killing all 44 aboard, occurred after the Oakland control tower heard the pilot screaming, "I'm shot! I'm shot! Oh my God, help!" An FBI investigation discovered that Frank Gonzalez, 27, former member of the Philippine Olympic yachting team, owned the .357 Smith & Wesson revolver found in the wreckage and had purchased a $100,000 insurance policy before boarding the flight.

California-based investigators from the National Transportation Safety Board reached the PSA crash site Monday night and today were joined by a team from Washington that was headed by vice chairman Goldman. Investigators said they were looking for a weapon as well as the flight data recorder among the debris.

The cockpit voice recorder, found Monday night and sent to Washington, "should give us a very good idea of what happened the last 30 minutes of that flight," Bretzing said.

PSA pilots often refer to the BAe 146 as "the killer bee," -- alluding to its odd, low-bellied design -- but they say it is a steady, "forgiving" aircraft that is easy to fly.

In service for only three years, the plane and its engines, made by Connecticut-based Avco Lycoming, have not accumulated the lengthy flying record needed to judge reliability, but so far PSA -- the first major U.S. airline to use the aircraft -- has reported only one other serious mishap. An engine failed during a February flight, sending one chunk of metal into the cabin and forcing an emergency landing but causing no injuries. Staff writer Nell Henderson in Washington and special correspondent Matt Lait in Los Angeles contributed to this report.