The White House acknowledged for the first time that it might not be able to close the US military prison at Guantanamo Bay by January as President Barack Obama promised.
Senior administration officials said difficulties in completing the lengthy review of detainee files and resolving thorny legal and logistical questions mean the president's self-imposed January deadline may slip.
Mr Obama remains as committed to closing the facility as he was when, as one of his first acts in office, he pledged to shut it down, said the officials, who spoke on condition of anonymity in order to more freely discuss the sensitive issue.
They said the White House still was hoping to meet the deadline through a stepped-up effort.
The prison in Cuba was created by former President George Bush after the September 11, 2001, terrorist attacks as a landing spot for suspected al Qaida, Taliban and foreign fighters captured in Afghanistan and elsewhere.
But it has since become a lightning rod of anti-US criticism around the globe. There are approximately 225 detainees still being held at the prison.
Mr Obama promised soon after taking office - and many times since - to close the prison, arguing that doing so is crucial to restoring America's image in the world and to creating a more effective anti-terror approach.
But eight months after Mr Obama's pledge and with only four months to go before the January deadline, a number of difficult issues remain unresolved.
They include establishment of a new set of rules for military trials, finding a location for a new prison to house detainees and finding host countries for those who can be released.
This has prompted top Republicans in Congress to demand that the prison stay open for now, saying it is too dangerous to rush the closure.
Even Democrats defied the president, saying they needed more information about Mr Obama's plan before supporting it. Congress is for now denying Mr Obama funds to shut down Guantanamo.