Chinese state-sponsored hackers breached a U.S. Navy contractor and stole huge amounts of highly sensitive intelligence involving secretive military operations, The Washington Post reported Friday.

Hackers acting on behalf of Beijing stole the data in January and February after breaking into an unidentified contractor that works for the Naval Undersea Warfare Center in Newport, Rhode Island, U.S. officials said on condition of anonymity, the newspaper reported.

Compromised in the breach were over 600 gigabytes of data involving “Sea Dragon,” a classified effort led by the Pentagon’s Strategic Capabilities Office, as well as signals and sensor data, submarine radio room information, the Navy submarine development unit’s electronic warfare library and sensitive plans to develop a supersonic anti-ship missile, the report said.

Officials said both the FBI and Navy are investigating the breach, according to the Post.

“We treat the broader issue of cyber intrusion against our contractors very seriously,” Navy spokesman Cmdr. Bill Speaks told The Post. “If such an intrusion were to occur, the appropriate parties would be looking at the specific incident, taking measures to protect current information and mitigating the impacts that might result from any information that might have been compromised.”

The spokesman said “it would be inappropriate to discuss further details at this time,” The Post reported.

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