Szymany airport is said to be where CIA interrogations took place
Prosecutors in Poland are investigating allegations that the American CIA interrogated terrorist suspects at a secret jail on Polish soil.
The investigation began three weeks ago, one prosecutor told Polish media.
A report released in June 2007 by the Council of Europe said the CIA ran secret jails in Poland and Romania in 2003-2005, to hold al-Qaeda suspects.
Polish and Romanian officials denied the claim. The secret CIA operation was known as "extraordinary rendition".
Asked about the alleged CIA jail in Poland on Monday, a Polish justice ministry spokesman told the AFP news agency: "I can confirm that such an investigation is ongoing".
Dick Marty, the Swiss senator who led the Council of Europe inquiry, said a secret agreement among Nato allies allowed the CIA to operate the secret jails.
Unnamed CIA sources quoted by Mr Marty said Poland was the "black site" where eight "high-value detainees (HVDs)" were interrogated, including Khalid Sheikh Mohammed - alleged mastermind of the 11 September attacks on the US in 2001.
An investigation by the BBC's Nick Hawton in 2006 identified the "black site" as Szymany airport, a remote military airstrip in north-eastern Poland.
The first reports about a Polish "black site" for CIA interrogations surfaced in 2005.