Japan has ordered its military to tighten security for the huge US bases scattered across the country.
US Marines also went on maximum alert after today's attacks on the World Trade Center and the Pentagon.
Only last Friday, the US embassy in Japan warned its citizens it had received unconfirmed information about a possible attack against US military installations or places in Japan frequented by US military personnel.
There are about 48,000 American military personnel in Japan, according to the embassy. The Japanese Justice Ministry said about 40,000 US civilians are registered as residents.