The soldier accused of downloading a huge trove of secret data from military computers in Iraq appears to have exploited a loophole in Defense Department security to copy thousands of files onto compact discs over a six-month period. In at least one instance, according to those familiar with the inquiry, the soldier smuggled highly classified data out of his intelligence unit on a disc disguised as a music CD by Lady Gaga. Criminal charges were filed this week against Pfc. Bradley E. Manning, 22, who was accused of downloading more than 150,000 diplomatic cables, as well as secret videos and a PowerPoint presentation. Since his arrest in May, with initial accounts blaming him for leaking video of a deadly U.S. helicopter attack in Baghdad in 2007, officials have sought to determine how he could have removed voluminous amounts of secret data without being caught. A Defense Department directive from November 2008 prohibits the use of small thumb drives or larger external memory devices on any of the estimated 7 million computers operated by the Pentagon and armed services. The order was issued to forestall the accidental infection of national security computer networks by viruses -- and the intentional removal of classified information. Defense Department computers have their portals disabled to prevent the use of external memory devices that are ubiquitous in homes, offices and schools, officials said. A recent amendment to the order allows the rare use of thumb drives, but only with official approval as required by a current mission. But the Pentagon directive and the amendment did not ban the use of compact-disc devices, which are built into many computers and therefore not included in the prohibition against the use of external memory devices. According to Pentagon officials and one former hacker who has communicated with Manning, he appears to have taken compact discs that can accept text, video and other data files into an intelligence center in the desert of eastern Iraq to copy and remove the classified information. At a Pentagon news conference Thursday, Defense Secretary Robert M. Gates and Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said they would reserve judgment on whether to order a sweeping review of security measures until it was determined whether the actions of which Manning is accused represent a broader problem. 