A US court has found a Chinese-born engineer guilty of conspiring to export sensitive defence technology to China.
Chi Mak, 66, was also convicted of acting as a foreign agent and of making false statements to federal agents.
Prosecutors said FBI agents found CDs containing encrypted information about defence projects in the luggage of two relatives at Los Angeles airport.
They also alleged that he gave thousands of documents to his brother, who handed them to Chinese authorities.
Mak said during the six-week trial that he copied sensitive documents from his employer, defence contractor Power Paragon, and kept copies in his office.
Mak, a naturalised US citizen, said he had not realised at the time that making copies was illegal.
He is to be sentenced on 10 September and faces up to 35 years in jail.
Assistant US Attorney Greg Staples said the government may use the guilty verdict to try to negotiate plea bargains with other members of Mak's family.
Mak's wife, brother and other relatives are awaiting trial later this month.
Mak was arrested in October 2005 in Los Angeles, California, after FBI agents stopped his brother and sister-in-law as they boarded a flight to Hong Kong.
Investigators said they found three encrypted CDs in their luggage that contained documents including one on a propulsion system that could make submarines virtually undetectable.
Mak had pleaded not guilty to all charges. He was not formally charged with espionage as the information was not officially deemed to be classified.