Two Arabic translators and a Muslim chaplain face charges ranging from espionage to adultery at the base, where interrogators are questioning some 660 detainees from 44 countries.
U.S. Army Col. Jack Farr was charged Saturday with "wrongfully transporting classified material without the proper security container on or around Oct. 11," and lying to investigators, said a statement from the U.S. Southern Command.
Spokesman Lt. Commander Chris Loundermon, speaking from the command's headquarters in Miami, said he did not know if Farr had direct contact with detainees and would not describe the classified material.
Farr is a reservist who had been on temporary duty at Guantanamo Bay for six months and had left to return to his home state, which Loundermon did not know.
"He was departing when the investigation revealed that he had some security violations," Loundermon said. "He voluntarily came back."
Farr is not under arrest and has not been suspended, Loundermon said: "He didn't present a flight risk and he was not likely to engage in any further serious misconduct."
Farr's charges have been forwarded to the base commander, who could dismiss them, refer them to a court-martial or direct a pretrial investigation.
Two Army lawyers appointed to represent Farr could not be reached Saturday.
Security has been tightened at Guantanamo Bay since the first arrests were announced in September and military investigators arrived.
New measures include firewalls on computer systems, increased bag screening and inspection of workers' electronic equipment before the leave the remote base on Cuba's eastern tip, which can only be reached by aircraft chartered by the military.
The first person arrested was Senior Airman Ahmad I. al-Halabi, an Air Force translator arrested July 23 at Boston's Logan International Airport when he returned from a visit to Egypt.
A naturalized U.S. citizen, al-Halabi worked as an Arabic translator and is accused of collecting secrets about the base and messages from prisoners with plans to transmit them to an unspecified enemy in his native Syria.
He has pleaded innocent to 32 charges, including espionage and aiding the enemy. The most serious charges carry a possible death sentence.
Officials did not announce al-Halabi's arrest until news broke of the Sept. 10 arrest of Army Capt. Yousef Yee, the Muslim chaplain at Guantanamo Bay who is a Chinese-American native of Springfield, New Jersey.
Federal agents said Yee, who converted to Islam after graduating from West Point, was found carrying sketches of the military prison at Guantanamo Bay, where he counseled prisoners and advised the detention mission commander about Islam and its culture.
Rumors grew that Yee, 35, would be charged with espionage, but instead he was charged Oct. 10 with disobeying a general order by taking classified material home and transporting it without proper security.
On Tuesday, the military released him, but at the same time pressed additional charges of storing pornography on a government computer and committing adultery, an offense under the military code.
Yee, who has pleaded innocent, faces a preliminary hearing Monday at Fort Benning, Georgia.
The third arrest came Sept. 29, also at Logan Airport. Customs agents found 132 compact discs, including one with hundreds of classified documents labeled "SECRET," in the luggage of Ahmad F. Mehalba, who was a civilian interpreter at the base.
Mehalba, 31, was arrested as he arrived from his native Egypt. The government on Nov. 12 charged him with gathering defense information and lying to federal investigators.
Mehalba says he is innocent.
Officials have said they have had difficulty finding enough translators with security clearance to interpret at the interrogations of detainees, who are mainly Muslims. Some speak Arabic but many — since most prisoners were arrested in Afghanistan
speak Pashto and Dari.
The United States has defended its prolonged detention of the detainees, who are suspected of links to Afghanistan's fallen Taliban regime and al Qaeda terrorist network, saying they still are providing important intelligence.
Not all of it has been high quality. Officials acknowledged this week that more than 10 detainees moved to a medium-security block as a reward for useful information have been returned to high security cells in the past three months because they passed along bad information or behaved badly.