In late 2005, U.S. government officials, invited by Japan, observed a counterterrorism drill at the Mihama nuclear plant in Fukui Prefecture and came away worried about the security situation at the complex.

Some 2,000 police officers, firefighters, nuclear power officials, local authorities and residents were involved in the exercise, in which the plant comes under attack by foreign terrorists who entered the harbor by boat.

The focus of much of the drill was on evacuating residents, but the U.S. officials were more concerned about what they felt were security gaps. This led Washington to push Tokyo to pay more attention to physically protecting the facilities.