Authorities in Tokyo are concerned that if Islamic terrorists wanted to stage a 'spectacular', the best day to try it would be on election Sunday. Not only would it strike a blow against a country that has committed troops to Iraq, but falling on September 11 would make it an echo of the attacks against the United States four years ago. It has been a decade since the last serious terrorist threat to Japan's security, when members of the Aum Shinrikyo cult released sarin nerve gas on Tokyo's subway system, killing 11 people, but a series of recent incidents has left the authorities and residents jumpy. Bomb threats were made against 11 hospitals in the capital this week and a Japan Airlines Express aircraft was evacuated at Sendai Airport last week after a telephone caller said a bomb was aboard. Security at Prime Minister Junichiro Koizumi's official residence was also tested recently when a woman drove through a barrier and tried to commit suicide by ritual disembowelment. 'It is impossible to avoid a terrorist attack if the people who are carrying it out are determined,' said an official of the Tokyo Metropolitan Government. 'Personally speaking, I am quite concerned that there could be such an attack in Tokyo because we cannot count on security here.' Security has been stepped up in Japan after bombings on buses and trains in London in July. In the countdown to the election, 13,600 officers have been posted at train stations and public facilities, such as hospitals, while sniffer dogs are searching numerous locations. 'I think most Japanese people are concerned about security now because we know that taking a bus or train is not like going through the security before someone gets on an aircraft,' said the Tokyo official. 'In big cities, security on mass transport systems is weak.' And there is good reason for the concern; al-Qaeda issued a warning shortly after Self-Defence Forces personnel were committed to Iraq on a humanitarian mission that Japan had made itself a target. 'It's not a question of whether Japan will become a target of terrorism, it's a question of when,' said Katsuya Okada, president of the opposition Democratic Party of Japan.