The computers of a US Navy contractor were breached by Chinese hackers, who stole hundreds of gigabytes of information about secret projects, reports The Washington Post.

According to officials who spoke with the Post, the breaches occurred in January and February of this year. The target was an unnamed contractor that worked with the Naval Undersea Warfare Center, which the Navy uses to conduct “research, development, test and evaluation, engineering and fleet support center for submarines, autonomous underwater systems and offensive and defensive weapons systems associated with undersea warfare.”

Hackers stole 614 gigabytes of data from the contractor, relating to a project called Sea Dragon, a secret project that the Defense Department explained as a new “disruptive offensive capability” being integrated onto “an existing weapons system with an existing Navy platform.” The Post describes the project as a “supersonic anti-ship missile” to be used aboard submarines. The project began in 2012, and was to begin testing in September 2018. The Post notes that in addition to information about Sea Dragon, hackers stole “signals and sensor data, submarine radio room information relating to cryptographic systems, and the Navy submarine development unit’s electronic warfare library.”

In his book Cybersecurity and Cyberwar: What Everyone Needs to Know, New America Strategist P.W. Singer noted that China has stepped up its cyber espionage efforts in the last decade, with the People’s Liberation Army setting up units dedicated to cyber attacks and espionage, and encouraging “patriotic hackers” to help put it on even footing with the superpowers that it now competes with — charges that China vehemently denies.

This new breach comes at a time amidst new tensions between the US and China, The latter has worked in recent years to expand its sphere of influence, laying claim on the South China Sea — part of the Pacific Ocean bracketed by China, Vietnam, and the Philippines, performing military exercises and building artificial islands to help support its forces and to bolster its territorial claims. These actions have led to increased tensions between with its neighbors and the US, which has its own naval presence in the region, something that China has protested. Given that the US operates in the area, the theft of secrets of naval technology can be problematic if it’s used to develop defensive countermeasures or to create a comparable weapons system.

This isn’t the first time that government-backed Chinese hackers have been implicated in the theft of data from the US military. Chinese hackers were also implicated in a 2009 breach that compromised Pentagon’s F-35 Joint Strike Fighter project, stealing terabytes of information related to the plane’s design. That’s not the only breach that the F-35 project has seen: in 2014, a Chinese national named Su Bin was arrested and charged with conspiring to break into defense contractor systems and allegedly stole information related to the C-17 transport plane, and the F-22 and F-35 fighter jets. (He later pled guilty.)

Chinese spies are believed to have compromised numerous other weapons systems as well, including the Patriot missile system, the Terminal High Altitude Area Defense (THAAD), the Navy’s Aegis ballistic missile defense system, and more. In some cases, China developed systems that bore striking similarities to their US counterparts. In 2014 and 2015, Chinese hackers broke into the US government’s Office of Personnel Management, compromising the personal data of more than four million government workers.