``We received a letter from the suspect on June 21,'' a  Transport Ministry official told Reuters. ``We cannot comment on  the contents of the letter but it was concerning airport  security,'' he said.  

The knife-wielding suspect, an unemployed 28-year-old male,  forced his way into the cockpit of the All Nippon Airways plane  carrying 517 people on Friday and attacked the pilot.  

Local media reported the man, who was eventually overpowered  by airline crew, told police he tried to take the aircraft's  controls because he enjoyed computer flight simulation games and  wanted to try the real thing.  

NHK television reported the two page letter to transport  authorities gave a detailed account of how easy it was to hijack  a plane, and said the suspect also made calls to the airport to  point out security lapses.  

The official said the ministry investigated the issues  raised in the letter, but declined to give details.  

It was still unclear how the hijacker smuggled the 20 cm (8  inch) knife through security checks, amid unconfirmed reports  the weapon was made of ceramic.  

But an ANA official said the knife, which officials earlier  said was 30 cm (12 inches) long, would have been spotted on  X-ray no matter what it was made of because of its shape.  

The hijacker, whose name has not been released, was  described as a jittery man wearing soiled gloves who forced a  flight attendant at knifepoint to let him into the cockpit.  

The suspect had used a false name to board the flight and  was not on the passenger list, officials said.  

Passengers interviewed by reporters said most of them,  especially those on the lower deck, were unaware of the deadly  drama that was unfolding aboard the packed flight.  

No passengers or crew apart from the pilot were hurt in the  incident.  

Domestic media reports said the man, who had demanded that  the plane fly to a U.S. Air Force base in Yokota in western  Tokyo, told investigators he wanted to fly under Tokyo's scenic  Rainbow Bridge and make a loop, an idea he apparently got from a  computer piloting game.  

Transport authorities met with airline industry and airport  security representatives late on Friday and directed them to  ratchet up security.  

It also called on airlines to make sure their personnel  review proper procedures for hijackings as well as scheduling  another round of airline security talks for Saturday.  

The Boeing 747-400 had taken off from Tokyo's Haneda airport  Friday morning and was heading for Hokkaido, in northern Japan,  before turning around and landing back in Tokyo an hour and 19  minutes later.  

Shortly after takeoff, the plane sent the following message:  

``Emergency situation. Hijack. Hijack,'' according to a  flight transcript.  

``Affirmative, Affirmative,'' Haneda controllers responded  seven seconds later.  

The plane suddenly lurched after the hijacker stabbed the  pilot and apparently released the autopilot and tried to steer  the plane himself.  

The co-pilot, who was earlier forced out of the cockpit by  the suspect, and several off-duty pilots on board raced back  into the cockpit to wrest back control of the plane.  

They subdued the hijacker, using neckties, ropes and belts  to pin him down and tie him to a seat, while one of the pilots  landed the plane.  

ANA pilot Naoyuki Nagashima, a 51-year-old father of two,  was the first fatality in the 20 Japanese hijackings on record.  There were no other injuries among crew or passengers.  

The last hijack incident in Japan was in January 1997 when a  knife-carrying man demanded an ANA plane on a domestic route be  flown abroad. The man was subdued by crew and passengers and the  flight landed safely with no one being injured.  

After that, authorities ordered upgrades of the luggage  inspection monitors, but only five of the 19 units at Haneda had  been replaced, Transport Ministry officials said.