A Council of Europe investigator says he has evidence to prove the CIA ran secret jails in Poland and Romania to interrogate "war on terror" suspects.
Unusual flights arrived at Poland's Szymany airport in 2003
Dick Marty, a Swiss senator, has been investigating CIA operations on behalf of the European human rights body.
In his new report, released on Friday, Mr Marty says secret CIA prisons "did exist in Europe from 2003 to 2005, in particular in Poland and Romania".
The governments of both countries have strongly denied any involvement.
A spokesman for the CIA told the BBC that "the CIA's counter-terror operations have been lawful, effective, closely reviewed, and of benefit to many people - including Europeans - by disrupting plots and saving lives".
Mr Marty says he drew on multiple sources and used his own intelligence methods to investigate the CIA's "extraordinary renditions", the process under which terror suspects were transported around the world for interrogation.
"Some European governments have obstructed the search for the truth and are continuing to do so by invoking the concept of 'state secrets'.... This criticism applies to Germany and Italy, in particular," he said.
His report came as the first criminal trial over the CIA "extraordinary renditions" opened in Italy. Twenty-five CIA agents and a US Air Force colonel are on trial in their absence, accused of kidnapping an Egyptian terror suspect and sending him to Egypt, where he was allegedly tortured.
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 9/11 attacks on the US in 2001 - and Abu Zubaydah, a suspected top al-Qaeda operative.
The report says Romania "was developed into a site to which more detainees were transferred only as the HVD programme expanded".
"The secret detention facilities in Europe were run directly and exclusively by the CIA. To our knowledge, the local staff had no meaningful contact with the prisoners and performed purely logistical duties," the report continues.
But it adds: "the highest state authorities were aware of the CIA's illegal activities on their territories".
US President George Bush admitted in September that terror suspects had been held in CIA-run prisons overseas, but he did not say where the prisons were located.
Mr Marty says there is evidence that the CIA operations took place in line with Nato authorisations agreed on 4 October 2001, "some of which are public and some of which remain secret".
Some detainees were held in secret for several years and subjected to "degrading treatment and so-called 'enhanced interrogation techniques' (essentially a euphemism for a kind of torture)," Mr Marty says.
In a preliminary report last year Mr Marty said the CIA ran a "global spider's web" of secret flights.
In comments published in the French daily Le Figaro on Friday, Mr Marty said "suspected terrorists" were also "kidnapped then tortured and detained illegally in rogue states like Syria, where there is no civilian law or law governing the rules of war".
In January a European Parliament committee approved a report which said EU states knew of secret CIA flights over Europe.
Dick Marty is an experienced international lawyer and politician
The report said the governments also knew of the abduction of terror suspects by US agents and the US's use of clandestine detention centres.
A BBC investigation last year revealed that a well-known CIA Gulfstream plane, the N379P, had made several landings at Szymany airport in northern Poland in 2003.
The airport's flight log also showed that a Boeing 737 had flown direct from Kabul to the airport, which is not far from a Polish intelligence base in the village of Stare Kiejkuty.