The U.S. reportedly believes the torpedo that sank South Korea’s “Cheonan” warship was manufactured at the "January 18th" factory in Gaecheon in North Korea's South Pyeongan Province.

Japan’s Sankei Shimbun says that based on their own intelligence network and data from within North Korea, U.S. authorities believe the torpedo's propeller was made at the North Korean plant two years ago.

The paper quoted multiple Tokyo officials as saying that the U.S. conveyed this information to South Korea and Japan in June.

Sankei says Washington has strengthened surveillance on the January 18th plant, which is also known to produce ballistic missile engine parts.