Hackers linked to China’s government broke into one of the U.S. government’s most sensitive computer networks, breaching a system used by the White House Military Office for nuclear commands, according to defense and intelligence officials familiar with the incident…

U.S. officials familiar with reports of the White House hacking incident said it took place earlier this month and involved unidentified hackers, believed to have used computer servers in China, who accessed the computer network used by the White House Military Office (WHMO), the president’s military office in charge of some of the government’s most sensitive communications, including strategic nuclear commands. The office also arranges presidential communications and travel, and inter-government teleconferences involving senior policy and intelligence officials.

“This is the most sensitive office in the U.S. government,” said a former senior U.S. intelligence official familiar with the work of the office. “A compromise there would cause grave strategic damage to the United States.”

Security officials are investigating the breach and have not yet determined the damage that may have been caused by the hacking incident, the officials said.