The military told journalists on Wednesday its own staff may have accidentally given his lawyer classified documents.
Captain James Yee had been due to face a military version of a grand jury hearing in Fort Benning, Georgia on Tuesday.
He is accused of wrongfully taking classified materials at the US naval base in Cuba where 660 suspected al-Qaida and Taliban fighters are detained.
But the hearing was postponed to Monday after the government found that a handwritten note by Yee, included in a packet of materials sent to his lawyer, may itself have contained classified information.
No defence case
Petty Officer Christopher Sherwood of the Southern Command said the discovery led investigators to immediately notify security officials at Guantanamo Bay, prompting the hearing to be set back.
Yee's lawyer, Eugene Fidell, said the government sent four people to his office on Tuesday to collect the 15 pages of suspect documents, which included the chaplain's calendar.
“If I sound a little baffled, it's because I am," Fidell said, adding that he had encouraged the government to reconsider the public interest in pursuing the case.
“It seems to me that it makes it a lot more difficult to prosecute the case, since I don't understand how a chaplain is to handle it."
“If I sound a little baffled, it's because I am. It seems to me that it makes it a lot more difficult to prosecute the case, since I don't understand how a chaplain is to handle it"
James Yee defence lawyer
Yee is among four men having had contact with the prisoners at Guantanamo to face charges over security breaches at the base.
Other Guantanamo cases
US army Colonel Jack Farr was charged on Saturday with mishandling a citizen of Egyptian descent who worked at Guantanamo as an interpreter.
Farr was charged last month with gathering, transmitting or losing defence information and making false statements.
An air force translator, Senior Airman Ahmad al-Halabi, was arrested 23 July on charges of espionage and aiding the enemy by attempting to send information about the prisoners and the facility to Syria.
But Yee had been held for 76 days before being sent to Fort Benning for the hearing.
He was released on 25 November, when the military brought additional charges of making a false statement and adultery.
The 35-year-old is a graduate of the West Point military academy. He converted from a Lutheran religion to Islam and was a counsellor to Muslim detainees at Guantanamo.