Poland has announced a formal inquiry into claims that the US intelligence agency, the CIA, operated secret prisons or interrogation centres on its territory.
The Polish government has for weeks denied the existence of the alleged facilities.
Earlier this week, Poland said it would co-operate with a separate investigation led by the Council of Europe.
Turkey has also denied that the CIA interrogated suspected terrorists on Turkish soil.
Washington has so far declined to comment on the prison reports, which have caused controversy on both sides of the Atlantic.
Poland is a leading ally of the US in Europe, where it angered EU heavyweights Germany and France by sending troops to join the US-led war in Iraq.
