The National Transportation Safety Board ruled officially Thursday that the 1987 crash of a Pacific Southwest Airlines jet was caused by an attack by a gun-wielding passenger.

PSA Flight 1771 crashed Dec. 7, l987 about l75 miles northwest of Los Angeles, killing all 43 people aboard, including a former USAir employee who had opened fire with a revolver in a revenge attack aimed at a former supervisor who also was on board.Within weeks of the crash, authorities disclosed that the former airline employee, David Burke, had smuggled a .44-caliber Magnum revolver through a security checkpoint at Los Angeles International Airport when boarding the flight to San Francisco.

A cockpit recorder tape from the plane included gunshots in or near the cockpit, according to the NTSB. The board said in a brief summary of findings that the tape revealed "the sounds of a scuffle and several shots which were apparently fired in or near the cockpit."

Both the pilot and co-pilot were presumed shot.

According to the NTSB findings, the official probable cause for the accident was "interference" and "intentional sabotage" by an emotionally distraught passenger.

The safety board cited as contributing factors inadequate security and inadequate operating procedures by the airline that permitted Burke to bypass security at the Los Angeles airport because of his previous employment.

The accident prompted the Federal Aviation Administration to order a tightening of security measures taken at passenger checkpoints so that there is a closer scrutiny of airline and airport employees.