The Obama administration will begin a review of the classified files of the remaining 250 detainees at Guantanamo Bay immediately after taking office, as part of an intensive effort to close the U.S. prison in Cuba, according to people who advised the campaign on detainee issues.

Announcing the closure of the controversial detention facility would be among the most potent signals the incoming administration could send of its sharp break with the Bush era, according to the advisers, who spoke on the condition of anonymity because they are not authorized to speak for the president-elect. They believe the move would create a global wave of diplomatic and popular goodwill that could accelerate the transfer of some detainees to other countries.

But the advisers, as well as outside national security and legal experts, said the new administration will face a thicket of legal, diplomatic, political and logistical challenges to closing the prison and prosecuting the most serious offenders in the United States -- an effort that could take many months or longer. Among the thorniest issues will be how to build effective cases without using evidence obtained by torture, an issue that attorneys for the detainees will almost certainly seek to exploit.

Moreover, the new administration will face hard decisions regarding not just the current Guantanamo Bay detainees but also how it will handle future captures of terrorism suspects. It is unclear whether President-elect Barack Obama would consider holding some suspects without charge on national security grounds.

Although as a candidate Obama said he wanted to close the facility, his transition team has stressed that no decisions have been made "about where and how to try the detainees," said Denis McDonough, an Obama foreign policy adviser.