A police raid on Japan's second-largest motorcycle manufacturer Monday had nothing to do with two-wheeled vehicles. The authorities were looking for evidence that Yamaha Motor exported remote-controlled helicopters to China, in violation of Japanese law.

Police and government officials on Monday accused Yamaha Motor of illegally exporting unmanned crop-dusting helicopters, which could be converted to military use, to a Chinese company.

Japan's chief cabinet secretary, Shinzo Abe, says the authorities will conduct a thorough investigation.

Abe says it is regrettable that the helicopters, which he says could be used to transport weapons of mass destruction, were illegally exported.

Yamaha on Monday acknowledged exporting nine of its RMAX remote-controlled helicopters to China since 2001, through a Chinese company called Beijing BVE Technology.

Japanese media quoted police here as saying that Beijing BVE Technology is believed to be linked to the Chinese military.

Yamaha Motor director Toyoo Otsubo told a news conference that the company was surprised by the allegation.  Otsubo says the company thought it had followed proper export procedures, and says the Trade Ministry gave no indication that the helicopters could not be sold to China.

Otsubo says Yamaha Motor does not believe that it has broken any laws, but will cooperate fully with the investigators.

Police on Monday raided Yamaha Motor's headquarters and numerous other company locations.

Japan one year ago strengthened export controls on civilian unmanned aircraft that can carry more than 20 liters of liquids. That came amid rising global concern that terrorists might use such aircraft to spray chemical or biological weapons.

Yamaha Motor, a subsidiary of the leading musical instrument maker Yamaha Corporation, is one of the two major manufacturers in Japan of unmanned helicopters.