An ex-CIA officer has been held on suspicion of spying for China and crippling US intelligence operations.
Jerry Chun Shing Lee, a US citizen, was arrested on Monday after flying into New York from his home in Hong Kong and subsequently charged with unlawful possession of top secret information.
But the 53-year-old has long been suspected of a more serious crime: betraying US informants in China and dismantling a network that had taken years to build up.
He is accused of feeding information to the Chinese which caused the deaths or imprisonment of an estimated 20 American agents - one of the agency's worst intelligence breaches in recent years.
Lee's arrest follows an FBI investigation into the failings which started in 2012, two years after the CIA began losing its informants.
Investigators debated whether the closely-guarded secrets were breached as a result of a hack or a mole inside the CIA.
Mr Lee worked for the CIA between 1994 and 2007, when he left for Hong Kong.
He briefly returned to the United States in 2012 after reportedly being lured by a fake job offer.
During his stay, FBI agents searched his hotel rooms and found two small note books with details such as "true names and phone numbers of assets and covert CIA employees".
Mr Lee left for Hong Kong again in 2013.
It is unclear why Mr Lee decided to risk arrest by coming back to the US this month.
He remains in custody awaiting trial.