Hackers for the Chinese government compromised computers of a Navy contractor and stole highly sensitive data related to undersea warfare, according to U.S. officials.

The data stolen included secret plans for the Navy to develop a supersonic anti-ship missile, these officials told the Washington Post .

The January and February breaches targeted a contractor that works for the Naval Undersea Warfare Center, which is a military organization in Rhode Island that conducts research and development for submarines and other underwater weaponry.

The Chinese government hackers were able to take 614 gigabytes of material related to a project known as Sea Dragon, and also obtained signals and sensor data, submarine radio room information and the Navy submarine development unit’s electronic warfare library.

Although the stolen data included highly sensitive information that could be considered classified, the contractor was housing it on an unclassified network, which is raising other concerns with the Navy.

The Navy argued that the release of any other details about the compromised data could harm national security interest as the investigation into the hack is still ongoing.