Police and customs officials plan to open a criminal investigation into a used-car dealer from Fukuoka Prefecture on suspicion of illegally exporting a trailer truck that can be converted into a missile launcher to North Korea, sources close to the case said Friday.

The authorities are focusing on around five individuals, including a man who is believed to run the car dealership.

The sources said the company also attempted to export a large tractor unit to North Korea but the export application was turned down by the government in March. Exports of large tractors, which can be used to tow missile launchers, are subject to international trade controls and require government approval.

The case apparently illustrates Pyongyang's belief that Japan can be a source of equipment for its missile and weapons of mass destruction programs.