TOKYO (AP) _ The man who commandeered an All Nippon Airways  jumbo jet and fatally stabbed its pilot wrote a letter to the  Transport Ministry saying Japanese airlines were easy prey for  hijackers, an official said today.  

Yuji Nishizawa, who on Friday briefly took control of the plane  bound for the northern city of Sapporo, pointed out faults in  security measures at Japanese airports, said Transport Ministry  official Masanori Gennai.  

Gennai refused to disclose further details of the letter, but  national broadcaster NHK said that Nishizawa gave a detailed  description of how to carry a dangerous weapon on board a passenger  plane.  

The hijacker, a fan of flight simulation games, was subdued by  cabin crew after he took control of the jet, and the plane returned  safely to Tokyo's Haneda airport. None of the 503 passengers was  injured.  

The attack has led airports around Japan to bolster security and  upgrade surveillance levels from ``vigilance'' to ``strict alert,''  said ANA spokeswoman Hidemi Hayashi.  

Nishizawa's attack has already prompted some change.  

He managed to get the knife on board by taking it out of a bag  he had checked earlier in the day, when he flew from Osaka to  Tokyo. There was no security checkpoint between the baggage claim  area and the departure area of his next flight _ a loophole he had  pointed out in his letters.  

Ministry officials and airport and airlines representatives  discussed the loophole and have ordered guards to stop anyone  suspicious looking in that area, said ministry official Atsushi  Sugiyama. They are also considering changing the layout and adding  a metal detector.  

There have been 20 hijackings in Japan since 1970. Friday's was  the first to result in a fatality.  

Nishizawa, who suffered from depression and told police he  wanted to experience flying a real plane, was arrested shortly  after the jet landed.  

The most recent hijacking in Japan was on Jan. 20, 1997, when a  man armed with a kitchen knife commandeered a flight from Osaka to  the southern city of Fukuoka. None of the 192 people aboard in that  hijacking was injured.