A former US Army sergeant has been arrested for illegally retaining classified information and attempting to provide it to China's spy service.
Joseph Schmidt, 29, allegedly made numerous documents and Google searches which describe his attempts to provide US national security secrets to China.
His "efforts to betray our country", Washington state federal prosecutor Tessa Gorman said, were "shocking".
He was arrested at the San Francisco airport after arriving from Hong Kong.
Mr Schmidt was an active-duty soldier from 2015 to 2020, mostly serving at Joint Base Lewis-McChord in western Washington.
He primarily worked in intelligence and had access to Top Secret documents, the highest level of classified information in the US government.
Investigators say he conducted online research about how to defect to China and offered his services as an "interrogator" to the the Chinese consulate in Turkey and to China's intelligence service.
"I have a current top secret clearance, and would like to talk to someone from the Government to share this information with you if that is possible," he is alleged to have written in one email to the Istanbul consulate.
He is accused of creating a Microsoft Word document entitled "high level secrets".
Prosecutors say the title of the 23-page document was written in Chinese, but "nearly all of it is written in English".
Another document is titled "important information to share with Chinese government".
Web searches show that he consulted several spying forums on social media app Reddit, including "What Do Real Spies Do and How are they Recruited", and read one article titled "Ten Countries That Hate America Most".
After his army service ended, he moved to Turkey for around one month before moving to China. He did not return to the US until he landed in San Francisco on Friday.
"Members of our military take a sworn oath to defend our country and the Constitution," said Mrs Gorman, the acting US attorney for the Western District of Washington.
"In that context the alleged actions of this former military member are shocking - not only attempting to provide national defence information, but also information that would assist a foreign adversary to gain access to Department of Defense secure computer networks."
Mr Schmidt has been charged with attempting to deliver national defence information and retention of national defence information. He faces decades in prison if convicted.
It is not clear if he has hired a lawyer who can comment on his behalf.
The arrest comes just two months after two active duty members of the US Navy were arrested for spying for China in southern California.
Both men were allegedly paid thousands of dollars for blueprints and technical manuals and diagrams of ships and radar systems.