An engineer working for a defence contractor in California and two of his relatives have been charged with acting as agents of China.
Chi Mak, his wife, Rebecca Laiwah Chiu, and his brother, Tai Wang Mak, face up to 10 years in jail if convicted.
However, a federal court indictment did not charge them with the more serious crime of espionage.
All three were previously charged with conspiring to smuggle documents about a classified US Navy project to China.
They were held earlier this month at Los Angeles airport as they prepared to board a flight to China.
Mr Tai's wife, Fuk Heung Li, was also detained, but she has not been charged.
All the detainees are ethnic Chinese. Mr Chi and his wife are naturalised US citizens, while Mr Tai is a legal US resident.
The indictment - which was handed down by a grand jury on Tuesday - charges the three suspects with an additional count of "acting as agents of a foreign government".
An FBI affidavit filed with the original charges earlier this month said Mr Chi was a lead engineer on a research project which involves Quiet Electric Drive technology for US Navy ships.
Along with the two others, he is suspected of trying to smuggle sensitive information about the project while working for the California-based Power Paragon.
"Based on my experience and training, I believe the targets are foreign intelligence operatives," FBI special agent James Gaylord wrote in the affidavit.