Chinese government hackers compromised the computers of a U.S. Navy contractor and stole a large amount of highly sensitive data on undersea warfare, including plans for a supersonic anti-ship missile for use on U.S. submarines, The Washington Post reported on Friday, citing unnamed U.S. officials.

The breaches took place in January and February, the officials told the Post, speaking on condition of anonymity about an ongoing investigation led by the navy and assisted by the FBI.

The FBI did not immediately respond to a request for comment.