He pleaded guilty "to conspiring to gather and deliver national defense information to the People’s Republic of China":
As detailed in the plea agreement, in March 2003, while living in Hawaii, [he] applied for a job as a contract linguist in the FBI Honolulu Field Office. The FBI, aware of [his] ties to PRC intelligence, hired [him], as part of an investigative plan, to work at an off-site location where his activities could be monitored and his contacts with the PRC investigated. [He] worked for the FBI from August 2004 until October 2012.
AP:
During a sting operation, he accepted thousands of dollars in cash in exchange for past espionage activities, and he told an undercover FBI agent posing as a Chinese intelligence officer that he wanted to see the “motherland” succeed, prosecutors said.