The transport ministry confirmed that Yuji Nishizawa, who on Friday briefly took control of a flight from Tokyo to Sapporo, had warned that Japanese airlines were soft targets for hijackers.

A ministry spokesman refused to disclose further details of the letter, received on June 21, but national broadcaster NHK said that Mr Nishizawa gave a detailed description of how to carry a dangerous weapon on board a passenger plane.

News reports say the hijacker checked in under a false name and carried two kitchen knives onto the plane.

Flight ANA61, a Boeing 747 with 503 passengers and 14 crew on board, was flying from Tokyo's Haneda airport to the northern city of Sapporo on the island of Hokkaido.

About 25 minutes after take-off, 28-year-old unemployed Yuji Nishizawa started screaming at other passengers and cabin crew.

BBC Tokyo Correspondent Juliet Hindell: "Motive unclear"

Armed with a long knife he barged into the cockpit and - according to one report - demanded to be taken to Yokota, a US military airbase in western Tokyo.

He forced the co-pilot out of the cockpit and the plane turned back towards the capital.

One report said that the hijacker at one stage took the controls himself, but this was not confirmed by the authorities.

The pilot, Naoyuki Nagashima, died on the plane

The military airbase gave clearance for landing, but after about 20 minutes, the pilot and three other crew members overpowered the hijacker and he was restrained.

During the struggle, the pilot was repeatedly stabbed in the neck and shoulder by the hijacker. He was badly injured and died from his wounds soon after.

The pilot, Naoyuki Nagashima, 51, was pronounced dead by a doctor on board shortly after the plane landed in Tokyo one hour and 20 minutes after taking off.

Mr Nishizawa, who has been described as suffering from depression, told police he wanted to experience flying a real plane when he was arrested shortly after the jet landed.

All passengers were shocked but safe

Newspapers in Japan report that the man used to work as a part-time baggage handler at Haneda airport.

According to the report he also demanded to be compensated by the airport authorities for his investigations into security problems.

In a telephone conversation with an airport manager he suggested that extra security staff were hired and that such a job should be given to him, reports say.