FBI agents in Los Angeles have arrested four people for allegedly trying to smuggle US military secrets to China.
An FBI affidavit said they were charged with theft of government property, conspiracy and transportation of stolen goods, Reuters news agency reported.
The affidavit said one suspect worked for a California defence contractor.
Those arrested - two married couples - are all ethnic Chinese. Two of them are naturalised US citizens, while the other two are legal US residents.
Chi Mak and his wife Rebecca Laiwah Chiu along with Mr Chi's brother, Tai Wang Mak and his wife Fuk Heung Li, were held last week at Los Angeles airport as they prepared to board a flight to China.
The FBI affidavit unsealed earlier this week showed that all four were later charged in a US district court.
It said Mr Chi was a lead engineer on a research project that involves Quiet Electric Drive technology for US Navy ships.
Along with the three others, he is suspected of trying to smuggle sensitive information about the project.
"Based on my experience and training, I believe the targets are foreign intelligence operatives," FBI Special Agent James Gaylord wrote in the affidavit, according to Reuters news agency.