US President Barack Obama ordered the CIA to close its global network of secret detention centres on Thursday.
A senior administration official said that the executive order "states that the CIA shall close as expeditiously as possible any detention facility that it currently operates and shall not operate any such detention facility in the future."
Swiss MP Dick Marty, who spearheaded the Council of Europe investigations that sought to expose the existence of clandestine interrogation centres in eastern Europe and Africa, said that he now expects the truth to start trickling out.
Mr Marty said: "It's fair that European countries try to help find a solution to this problem because European countries actively, or at least passively, collaborated in the commission of unlawful acts in the name of this so-called war against terrorism.
"For some countries, things are going to become very embarrassing," he predicted, advising European governments that they would "do well now to tell the truth."
Mr Marty said that he "would be very surprised" if any CIA-run facilities were still operating in Europe, but he added: "In east Africa or Morocco I might assume there is something."
He said that Mr Obama's order granting Red Cross officials access to all secret facilities might prove critical in revealing their locations, both past and present.
Poland, which strongly backed former US president George W Bush's "war on terror," repeated denials of involvement.
Romania is strongly suspected of hosting CIA prisons and a Romanian intelligence official, speaking on condition of anonymity, said that Mr Obama's order had made Romanian authorities "worried."
"They are not going to say anything because it will make Romania look guilty," he said.
In Ireland, opposition Labour Party human rights spokesman Joe Costello said: "CIA planes involved in rendition refuelled at Shannon and were going to and from Guantanamo Bay from Shannon.
"We have not been able to inspect the contents of the planes to determine if there were detainees in them, so we were complicit in this illegal, abominable activity," Mr Costello observed.
Lawyer Wolfgang Kaleck, who sued the German government last June to demand that it pursue the extradition of 13 CIA agents sought in the kidnapping of German citizen Khaled al-Masri, said that Mr Obama's order could clear the way for more clarity into European governments' involvement.
Mr Kaleck said: "We believe, based on reports from human rights organisations, that there are still people detained through this programme."
See also: Britain: Closure 'is not enough'
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