Poland has announced a formal inquiry into claims that the US CIA operated secret prisons or interrogation centres on its territory.
Prime Minister Marcinkiewicz: "This matter must finally be closed"
Announcing the move, Prime Minister Kazimierz Marcinkiewicz said the issue had to be resolved.
The Polish government has always denied the existence of such facilities.
The BBC's Adam Easton in Warsaw says that over the last few days, the Polish government has come under increasing pressure to be seen to act.
Several newspapers have run successive front-page stories about the issue, with one quoting a spokesman for the US-based group Human Rights Watch saying that Poland had been the main base for interrogating terrorist suspects.
Another said the secret prisons were only closed down after the story first became public last month.
Mr Marcinkiewicz said the detailed investigation would look at all possible locations to determine if there was any evidence to support the allegations.
"This matter must finally be closed, because it could prove dangerous for Poland," he said.
Earlier this week, the prime minister said the country would open its doors to a separate investigation led by the Council of Europe.
Meanwhile, a senior US official has defended the country's treatment of terror suspects and the transfer of prisoners to third countries for interrogation.
State department senior legal adviser John Bellinger told the BBC that Washington sought reassurance in those countries that prisoners would not be tortured.
He said allegations that hundreds of suspects were sent around the globe to be tortured were "ludicrous".
Mr Bellinger said the US did practice rendition, by which some terror suspects were sent to a third country to be questioned.
But he added that even transferring a prisoner to a country which had been criticised over its human rights record was not a violation of international law.
"We as a state department have got problems with the human rights records of some countries... but this does not mean per se that you may not transfer a person to those countries," he told BBC Radio 4's Today programme.
Mr Bellinger insisted that if there were such questions, the US would seek reassurances that a prisoner would not be subjected to torture.
But he said some practises had been wildly exaggerated.
"Some of the allegations more broadly about all sorts of things are ludicrous, [like one] about hundreds of flights from European cities taking people to be tortured," he said.
He repeated that Washington did not condone or practice torture, but would not comment on whether some prisoners had been subjected to interrogation techniques such as waterboarding - when a suspect is made to feel that they are drowning - to extract information.
The lawyer said he could not comment on speculation about so-called enhanced interrogation techniques or the existence of secret prisons.