US President Barack Obama has ordered the federal government to buy a prison in Illinois to take a number of inmates from Guantanamo Bay.
The move is a key part of Mr Obama's plan to close the Cuba-based jail.
The number of inmates for transfer to the Thomson Correctional Center has not been given officially, but US media report it could be between 35 and 90.
Senior officials said security would be upgraded, making Thomson the most secure jail in the country.
Obama administration officials have said that closing Guantanamo Bay is "essential" in removing a key al-Qaeda recruiting tool.
Mr Obama had given himself one year to achieve this, but with officials still trying to work out what to do with about 215 inmates at the camp, he admitted in November that a 22 January deadline had slipped to later in 2010.
A letter signed by Secretary of State Hillary Clinton and Defence Secretary Robert Gates says: "The president has directed, with our unanimous support, that the federal government proceed with the acquisition of the facility in Thomson."
A briefing by senior administration officials said the move represented "an important step to achieve our national security objectives".
Thomson would "go beyond even maximum security prison standards".
An improved perimeter fence would make the prison "the most secure in the country", officials at the briefing said.
They said the number of Guantanamo inmates at Thomson would be "limited".
There remain questions on the legality of transfers to US soil but the briefing officials said it was permissible to bring in detainees for prosecution.
The briefing heard: "It would be a violation of current law to transfer individuals for anything but the purpose of prosecution - that's the [legal] change we'll be looking for."
There have been concerns in the US about inmates escaping or eventually gaining the right to live on American soil.
However the Clinton/Gates letter says: "The president has no intention of releasing any detainees in the United States."
Another letter, from Attorney General Eric Holder and Homeland Security Secretary Janet Napolitano to Congress, and obtained by the Associated Press news agency, says a detainee will be treated as though he is at the border trying to obtain entry, but will not do so.
No timeframe has been given for the first transfer.
The BBC's Adam Brookes, in Washington, says that the prison transfer might go some way to addressing the problem of Guantanamo but it will not solve it.
Our correspondent adds that European nations - who have until now taken the attitude that if the US will not house the detainees, they will not either - may be softening their stance to accept small numbers of prisoners.
The near-empty Thomson prison, about 150 miles (240km) from Chicago, was promoted as an alternative by Illinois Senator Richard Durbin.
The prison, built in 2001, has the capacity for 1,600 inmates, but due to budget constraints only houses 200 prisoners.
It would be sold to the Federal Bureau of Prisons and then part of it leased to the Department of Defense, reports say.
Federal prisoners would be held at the facility, as well as the former Guantanamo inmates, officials said.
Of the remaining Guantanamo inmates, some are expected to be sent to other countries, while others could face military tribunals or be tried in US courts.
Prisons in Colorado, Montana and elsewhere in Illinois had expressed an interest in housing Guantanamo inmates.
