Fourteen European states have colluded with the CIA in secret US flights for terror suspects, a report for Europe's human rights watchdog has concluded.
The seven-month inquiry was sparked by media allegations that the US intelligence agency ran secret jails abroad for terror suspects.
Reports claim al-Qaeda members are being held in clandestine jails
The US admits to picking up terrorism suspects but denies sending them to other countries to face torture.
BBC News examines the investigations, accusations and legal implications of the claims.
What have investigators found?
Swiss senator Dick Marty has reported to the Council of Europe that 14 European governments have colluded with the CIA over the transport of terror suspects around the world for interrogation - a practice known as "extraordinary rendition".
Spain, Turkey, Germany and Cyprus provided "staging posts" for rendition operations, while the UK, Portugal, Ireland and Greece were "stop-off points", the report said.
Italy, Sweden, Macedonia and Bosnia allowed the abduction of residents from their soil, it said.
Mr Marty also said there was evidence to back suspicions that secret CIA camps are or were located in Poland and Romania - allegations both countries deny.
His conclusions are based on air traffic logs, satellite photos and accounts of prisoners who say they were abducted.
The Council of Europe report comes less than two months after European parliament investigators said the CIA had carried out more than 1,000 undeclared flights over European territory since 2001.
Following the outcry over last November's media allegations, Spain, Sweden and Iceland looked into reports that CIA planes stopped in their territory while transporting terror suspects.
What were the allegations?
Claims that the CIA held al-Qaeda suspects in Eastern Europe, Thailand and Afghanistan were first reported in the Washington Post on 2 November 2005.
According to the newspaper, centres - known as "black sites" - were set up in the wake of the 11 September attacks on the US in 2001.
About 30 detainees, considered major terrorism suspects, were held in the sites, the paper said.
Thailand has issued a denial about its alleged jail, which the Washington Post says has been closed down.
At least 70 other detainees have since been handed over to intelligence services in Egypt, Jordan, Morocco, Afghanistan and other countries, according to the paper.
European media reports also alleged that the CIA has used several European airports for its programme of "extraordinary renditions".
What is 'extraordinary rendition'?
Under the highly secretive process, US intelligence agencies send terror suspects for interrogation by security officials in other countries, where they have no legal protection or rights under American law.
Some individuals have claimed they were flown by the CIA to countries like Syria and Egypt, where they were tortured.
In August 2005, human rights group Amnesty International called on the US to reveal details of an alleged secret detention of suspects abroad.
The group highlighted the case of two Yemeni men who claimed they were held in secret, underground US jails for more than 18 months without being charged.
During that time, they say they were tortured for four days by the Jordanian intelligence services.
How has the US responded?
The US government and its intelligence agencies maintain that all their operations are conducted within the law.
Washington does not deny that terror suspects have been transferred for interrogation in other countries, but rejects accusations that they are being tortured.
US Secretary of State Condoleezza Rice says all American interrogators are bound by the UN Convention on Torture, whether they work in the US or abroad.
The US has neither confirmed nor denied the alleged existence of secret CIA jails.
What are the legal implications?
The US says it is at war against terror, so criminal law does not apply and nor do the laws of combat, such as the Geneva Conventions.
European countries, however, have to respect the European Convention of Human Rights.
The Marty report says Council of Europe member states have violated the Convention in a number of ways:
- Secretly detaining a person for an indefinite period of time while failing to ensure procedural legal guarantees
- Capturing a person and handing the person over to the US for unlawful transfer to a US-administered detention facility
- Permitting the unlawful transportation of detainees on civilian aircraft carrying out rendition operations
- Passing on information or intelligence to the US where it was foreseeable that it would be used to carry out a rendition
- Participating in interrogation of people subject to rendition
- Accepting or making use of information gathered in the course of detainee interrogations, before during or after torture, or the threat of it
- Making available civilian airports or military airfields as staging points or stopover points for rendition flights