The families of six people killed in a mysterious aircraft crash in 1997 that sparked allegations of pilot suicide have lost their legal action against a Singapore-based airline.

Relatives claimed that SilkAir was negligent because it allowed the pilot, Tsu Way Ming, to fly although he was mentally unsound and had a history of safety breaches. But a Singapore High Court Judge, Tan Lee Meng, said in a judgment released yesterday that they failed to prove that Mr Tsu intentionally caused the crash.

The Singapore-bound Boeing 737-300 plunged into a jungle river shortly after leaving Jakarta on 19 December 1997. All 104 people on board died. SilkAir, the regional arm of the national flag carrier, Singapore Airlines, said there was no evidence that the pilot committed suicide by deliberately bringing the aircraft down.

Evidence that the aircraft's controls had been set in a "nose down" position and that the engines were on full power was dismissed by the airline, which said that the pilot might have been trying to descend rapidly because of an emergency, such as loss of cabin pressure. The data and cockpit recorders stopped working moments before the crash.

An Indonesian investigation concluded Mr Tsu was not suicidal. However, a separate investigation by the US National Transportation Safety Board found nothing wrong with the aircraft and said the accident "can be explained by intentional pilot action".

The board said Mr Tsu, once a stunt flyer with the Singapore Air Force, was in serious debt from speculating on the financial markets and had been reprimanded by SilkAir's management several times in the weeks before the crash. The United States was asked to participate in the investigation because the plane was made by Seattle-based Boeing.

Most of the relatives accepted compensation of $200,000 (£140,000) per victim and did not sue.

Thomas Oey, an American plaintiff who had lost his mother and brother in the crash, said he was "disappointed" by the outcome. "But I think the case brought a lot of information to light," Mr Oey said. "I'm glad of that."