(CNN) -- A former U.S. Army officer has been charged with passing information relating to nuclear weapons and other military secrets to his Chinese girlfriend, the Justice Department said.
Benjamin Bishop, 59, was arrested Friday at work and appeared in federal court in Honolulu on Monday.
He was formally charged with "willfully communicating national defense information to a person not entitled to receive such information."
Court documents identified the woman as a 27-year-old Chinese citizen. Her name was not disclosed and she has not been charged. She was in the United States legally on a student visa, authorities said.
Court documents do not accuse the woman of spying, but an affidavit notes that Bishop met her at a conference in Hawaii where she may have been attempting to target individuals with access to classified information.
Investigators say Bishop began a romantic relationship with her in June 2011 and sought to hide it from authorities.
Bishop, who had top-secret clearance, is also charged with unlawfully retaining classified documents in his home.
He will remain in custody pending a detention hearing on Friday. If convicted, Bishop faces up to 20 years in prison.
"The arrest of Mr. Bishop is just the first step in what is going to be a long process and we are actively continuing the investigation to determine the roles of all those involved in this case," U.S. Attorney Florence Nakakuni said.
Bishop's court-appointed lawyer, Birney Bervar, said the charges were devastating for her client.
"He has served his country honorably for 29 years and he maintains he would never do anything to intentionally harm the United States," Bervar said.
Government officials said Bishop had been in the Army reserves but was activated for about three years before being hired by a contractor.
After becoming suspicious of his activities, federal agents conducted wiretaps, physical and electronic surveillance, and searches of Bishop's home.
Authorities would not comment on the extent of potential damage to national security, but said the information involved "related to nuclear weapons, information on planned deployment of U.S. strategic nuclear systems, information on the ability of the U.S. to detect low and medium-range ballistic missiles of foreign governments, and information on the deployment of U.S. early warning radar systems in the Pacific Rim."