The CIA used a secret prison in Poland to detain and torture its key 9/11 suspect, it has been alleged.
Khalid Sheikh Mohammed, the attacks' alleged mastermind, was allegedly flown into the country and "waterboarded".
The allegation comes from a former human rights official amid an investigation into the country's role in secret CIA rendition flights.
The CIA told the BBC it would not discuss which detention facilities it may have used in the past.
But Dick Marty, the former Council of Europe Rapporteur on Torture, said there was now a "criminal standard of proof" to support long-held suspicions that the CIA had used Poland in a global network of detention sites for the most important al-Qaeda suspects.
Mr Marty told the BBC: "If I use the judicial standard of proof, and I used to be a magistrate, then I say, yes, Mohammed was in Poland. Yes, he was tortured."
His conclusions come amid investigations in Poland and neighbouring Lithuania into whether officials in each country broke the law in assisting the CIA's activities.
A BBC investigation has pieced together evidence supporting suspicions that the senior al-Qaeda planner was one of a number of high-profile suspects held at a secret facility in Poland.
Khalid Sheikh Mohammed, one of al-Qaeda's most senior figures, was seized in Pakistan in March 2003.
Within days he had been flown out of the country. According to a heavily-redacted report by the CIA Inspector General, the agency's internal watchdog, Mr Mohammed then received "183 applications of the waterboard."
Mr Mohammed was later interviewed in confidence by the International Committee of the Red Cross.
In that interview, which was subsequently leaked, the suspect described being waterboarded.
He was placed upside down on a vertical bed and a cloth was put over his mouth on to which water was poured. The subject feels like they are drowning as they struggle to breathe.
The fact that the CIA used the so-called "enhanced interrogation technique" is not in dispute - but the location of Mr Mohammed's interrogations has been shrouded in mystery.
Polish authorities have denied hosting rendition flights - but logs reveal that CIA flights landed at an isolated rural airstrip in Szymany, North East Poland, between December 2002 and September 2003.
In a BBC interview, Thomas Hammarberg, the Council of Europe's Commissioner on Human Rights, said he now believed detainees had been subjected to "intense torture" and called for officials who were complicit in the ill treatment to be prosecuted.
Polish prosecutors are examining whether officials were involved in wrongdoing but say their investigation will take months to complete.
In a statement to the BBC the CIA said "The programme is over. This agency does not discuss publicly where detention facilities may or may not have been."