The DOJ announced on Tuesday that a former CIA officer found with top-secret material including the names of CIA employees and informants had been arrested.

Current and former intelligence officials believe that Lee was passing the top-secret information to China to help its government identify CIA informants and employees operating in that country, at least 20 of whom were killed or thrown in jail.

Ex-CIA officer Jerry Chun Lee, who was found to be in possession of classified and top-secret materials long after he left the agency, was arrested on Tuesday after flying into John F. Kennedy Airport from Hong Kong, the Department of Justice said on Tuesday.

FBI agents found the materials — which included "true names and phone numbers of assets and covert CIA employees, operational notes from asset meetings, operational meeting locations and locations of covert facilities" — when conducting a search on his Hawaii hotel room in August 2012, according to a newly unsealed affidavit.

The documents in Lee's possession were photographed by FBI agents on August 15, 2012, when Lee and his family arrived in Virginia from Hawaii and checked into a hotel in Fairfax.

"A review of photographs taken during the August 13, 2012, search in Hawaii and the August 15, 2012, search in Virginia revealed that, during his stay in both hotels, Lee possessed two small books (the 'books') best described as a datebook and an address book," the affidavit said. "Each book contained handwritten notes. During both searches the books were located inside LEE's luggage in a small, clear plastic travel pack."

Lee maintained contact with his former CIA colleagues after he left the agency and was interviewed by FBI agents "on five separate occasions in or about May and June 2013 and never suggested that he possessed these books," the affidavit said.

Current and former intelligence officials believe that Lee was passing the top-secret information to China in an effort to help the Chinese government identify CIA informants and employees operating in that counry, at least 20 of whom were killed or thrown in jail.

Lee left the CIA in 2007 and Beijing began cracking down on the CIA's network in 2010, The New York Times reported last year. Between 2010-2012, at least 12 CIA sources were killed by Chinese government officials.

The intelligence community couldn't figure out who had betrayed the agency and the United States, if anyone — some were convinced there was a mole, while others believed that the Chinese had hacked the system by which the agency communicates with its foreign sources, according to The Times.

Lee is now believed to have been that mole. But there are other factors — including basic "sloppy tradecraft," the Times' report said — that could have contributed to the massive breach.

Lee had been living in Hong Kong but was promptly arrested upon his return to the US on Monday night. It is unclear what brought him back to the US, but he was lured back once in 2012 on the promise of a job offer, according to NBC. No charges were filed at that time, even as Lee was the subject of an intense counterintelligence investigation.

Read the full affidavit below: