National Security Adviser James Jones: 'This facility is going to be extremely secure'
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.
Paul Reynolds, BBC News Online world affairs correspondent
This is another important step on the way to closing Guantanamo Bay, but not the final one. Some 210 prisoners remain at Guantanamo.
Prisoners who will be transferred to the Thomson jail will be those who face either criminal trials or military tribunals, and probably another larger group against whom there is no useable evidence, but who are deemed too dangerous to release.
Military tribunals will be held at the newly designated jail. More than 100 other inmates whom the US would like to release would probably be held at Guantanamo until some country can be found to take them.
President Obama set 22 January 2010 as the target for closing Guantanamo. He has accepted this date will be missed and it is not clear when he will fulfil his promise.
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 officials said the prison would be "the most secure in the country".
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."
No timeframe has been given for the first transfer.
Republicans criticised the move. Senate Republican leader Mitch McConnell said Americans and Congress had "already rejected bringing terrorists to US soil for long-term detention, and current law prohibits it".
Human rights groups said the mere relocation of suspects continues to violate the legal principle that people cannot be held without charge or trial.
"The only thing that President Obama is doing with this announcement is changing the Zip Code of Guantanamo," Amnesty International said in a statement.
The BBC's Adam Brookes in Washington says the prison transfer might go some way to addressing the problem of Guantanamo but it will not solve it.
He says 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.
Thomson jail is tipped as the 'leading option' for Guantanamo inmates
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.
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