Japanese officials tightened security in Tokyo's trains and subways in the wake of yesterday's discovery of cyanide bombs in two stations that could have killed thousands of commuters. Bombs were found in Tokyo's Kayabacho and Shinjuku stations, and irritating fumes were reported in two others. The devices were discovered before their timers reached the moment of detonation. A similar device was found in Shinjuku station, the world's busiest, last month. Police immediately suspected that theAum Shinrikyo cult was behind the new attacks,but cult leaders denied any involvement.