The official, describing the apprehension at Boston's Logan International Airport, identified the suspect as Ahmed Mehalba. The official, who discussed the case on grounds of anonymity, said Mehalba had stopped in Boston Monday after arriving on a flight from Cairo.
An Air Force translator and a Muslim Army chaplain at the prison camp were detained earlier in the probe. After their arrests, Pentagon officials said that a third person was under investigation for possible security violations at the prison camp. It was not clear if Mehalba was this third person.
Melhaba was detained after agents with Immigration and Customs Enforcement noticed documents and computer disks that appeared to have come from the prison camp and that they suspected of being classified.
Unlike the two other arrests, which were planned, CBS News Correspondent Jim Stewart reports customs agents did not plan to stop Mehalba until he began acting nervous. The FBI was called in to interview Mahalba, who denied the documents were his, the official said.
After the interview, the FBI arrested Mehalba on charges of making false statements. He was being held in Boston and further charges are possible, said the official, who declined to describe the nature of the documents in Mehalba's possession.
About 660 suspected al Qaeda or Taliban members are imprisoned at the U.S. Navy base at Guantanamo. American officials are interrogating them for information on the terrorist network. The military has classified many details about the prison camp and the detainees and has not identified any of the men being held there.
The alleged spy cases have raised concern in the Defense Department about security at the base. For U.S. officials, the fear is Mehalba has already downloaded, sold or shared his information, as one of the other suspects, Senior Airman Ahmad I. al-Halabi, apparently tried to do.
Military authorities accuse al-Halabi, a 24-year-old translator, of sending e-mail with information about the prisoners at Guantanamo Bay "to unauthorized person or persons whom he, the accused, knew to be the enemy." The Air Force documents detailing the charges do not say who "the enemy" is.
When he was arrested, al-Halabi was carrying two handwritten notes from detainees that al-Halabi intended to turn over to someone traveling to Syria, the charging documents say. He also was carrying his personal laptop computer containing classified information about detainees and 180 messages from detainees he intended to send to Syria or Qatar, it was alleged.
Secret documents al-Halabi is accused of trying to pass to Syria include details of flights to and from the Guantanamo Bay base; names, serial numbers and cell numbers of prisoners; a map of the base; and other military documents.
A military attorney representing al-Halabi, Air Force Maj. James E. Key III, denied the charges. In Damascus, the Syrians denied that they had any links to the Air Force translator. Information Minister Ahmed al-Hassan said the accusations were baseless.
Al-Halabi is behind bars at a California Air Force base, facing 32 criminal charges. The most serious — espionage and aiding the enemy — could carry the death penalty.
The chaplain, Army Capt. Yousef Yee, has been held without charge since his Sept. 10 arrest. Yee was apparently detained because he was carrying several classified documents he was not authorized to carry, plus diagrams indicating which Guantanamo officials had interrogated individual prisoners and what questions they had asked.
Yee and al-Halabi knew each other, an Air Force spokesman said, but officials said they didn't know whether there had been any conspiracy to breach security at the prison camp. It was not clear if Mehalba knew either of the other suspects.