They were planning to strike at the World Cup 2002. From AP:
A senior al Qaeda member told U.S. authorities the group had plans to carry out attacks in Japan during the 2002 World Cup soccer tournament, local media reported yesterday.
U.S. authorities advised Japan of the information, which is believed to have come from the militant Islamic group’s third-ranking official, Khalid Shaikh Mohammed, the daily Sankei newspaper reported, citing anonymous sources. The Kyodo news agency carried a similar report.
The attacks were not carried out because al Qaeda did not have a network in Japan, which hosted the 2002 event jointly with South Korea, according to the paper.
The report said Mohammed was familiar with Japan. During a three-month stay in 1987, he reportedly studied rock-drilling machinery.