A Pentagon official said a member of the Navy working at Guantamano was being closely watched. A lawyer for some of the detainees and a former U.S. intelligence officer said two arrests were imminent. The sources spoke with The Associated Press on condition of anonymity.
Nearly two dozen investigators from the Miami-based U.S. Southern Command reported to the island over the past few days. Also reporting were five non-American-born interpreters contracted by the same company that employed an American translator who has been arrested.
Investigators will try to establish how a translator already under investigation got secret clearance and was allowed onto the base, and how a second translator managed to leave with classified information. In addition, a Muslim chaplain is under investigation after allegedly leaving with diagrams of the prison layout.
The translators, from San Diego-based Titan Corp., arrived as officials boosted security by closely monitoring e-mail messages, asking troops to report suspicious behavior, and postponing the assignment of another Muslim chaplain.
Army Maj. Gen. Geoffrey D. Miller, who commands the detention mission, said they also are increasing baggage checks and considering lie detector tests.
He thought Titan had done a good job but said its contract is under review: "They go through a very thorough screening process, but that contract is being reassessed."
"I was surprised" by the arrests, Miller said, but would not discuss how security might have broken down.
Titan employed Ahmed F. Mehalba, an Arabic translator charged with lying to federal agents when he denied the compact disc he was carrying contained secret information from Guantanamo.
A second translator, Senior Airman Ahmad I. al-Halabi, was already under investigation for allegedly making anti-American statements before he arrived at Guantanamo. He is now charged with espionage and aiding the enemy.
Both say they are innocent. The New York Times has reported investigators are reviewing tapes of the interrogations where suspected translators assisted, to see if they skewed the answers or questions to protect suspects.
Army Capt. Yousef Yee, the chaplain, is being held on suspicion of aiding the enemy.
Military and civilian officials acknowledge part of the problem is finding qualified linguists for Guantanamo, where about 70 translators help 200 interrogators in 17 languages.
"They're always looking for Arabic interpreters," said Peter Peterson, an Iraqi-American translator who arrived Tuesday. "I believe in what I'm doing, though, and I believe in the mission."
New leaflets at the base warn troops to watch what they say. Soldiers who a month ago would start up conversations with translators said they no longer do so.
"Now, you think twice about what you do," said Army Sgt. Jovani Barber, 24, of the U.S. Virgin Islands, who has been guarding detainees for two months.
Miller, a Texan, took command in November 2002 and quickly started rewarding detainees who cooperate in interrogations.
Rewards range from bottled water to being moved to a medium-security wing that Thursday held 120 detainees, according to Army Col. Gerry Cannon, who took charge of Camp Delta in August. Cannon said 130 detainees considered the least cooperative were in a separate section.
Miller has said the rewards system has yielded valuable information about terror cells and recruiting. "We have five times as much intelligence" as a year ago, he said Thursday.
But as restrictions eased, relationships reportedly grew between detainees and captors.
"Some of the men wrote to their families saying they had developed relationships with some of the camp personnel," said Qatari lawyer Najeeb al-Nauimi, who is trying to get at least 90 detainees released to their native countries.
Investigators want to know how deep the relationships were, and whether translators could have misrepresented statements to protect detainees.
"The biggest challenge has been the growth of the base, which has almost tripled in size," said Navy Capt. Leslie McCoy, who is in charge of the naval base. "But I am more concerned now."
Miller said officials continue to exercise security precautions because "These (detainees) are still very bad people."
The detention mission began Jan. 11, 2002, as an impromptu operation with 20 shackled terror suspects locked behind crude chain-link cells.
The prison camp — now an enclosed facility called Camp Delta — has grown to 660 detainees suspected of links to al Qaeda terror network or Afghanistan's ousted Taliban regime.
Construction workers said Thursday they have begun building a permanent concrete prison.
"You can have the most secure cells and an isolated military base, but if you don't control the people who come onto the base, you have a serious problem," Matt Levitt, a terrorism analyst and senior fellow at the Washington Institute, said by telephone.
Levitt said if there were three security breaches at Guantanamo it represented "a colossal intelligence failure."
The prisoners, from 42 countries, are not allowed access to lawyers and none has been charged. At least three are teenagers. Defense Secretary Donald Rumsfeld has said they could be held until the war on terror ends.
Earlier this year, President Bush named six men believed held at the base for possible trial by a military tribunal. Pentagon prosecutors and leaders must now decide what charges the men might face, and whether to go ahead with tribunals. The six men have not been named, and it is not clear what they are suspected of doing.