Dick Marty, a Swiss senator, said in a written report on Tuesday summarising his investigations so far: "Information gathered to date reinforced the credibility of the allegations concerning the transfer and temporary detention of individuals, without any judicial involvement, in European countries.
"Legal proceedings in progress in certain countries seemed to indicate that individuals had been abducted and transferred to other countries without respect for any legal standards," he added in his findings presented in Paris to a committee of the Council of Europe, the continent's human rights watchdog.
He is investigating the claims that the CIA transferred prisoners through European airports to secret detention centres, actions that would breach the continent's human rights principles.
Poland and Romania have been identified by the New York-based Human Rights Watch as sites of possible CIA secret prisons, but both countries have repeatedly denied any involvement.
Marty, in his report, added that it was "still too early to assert that there had been any involvement or complicity of member states in illegal actions".
He was critical of the US for not formally confirming or denying the allegations.
"Information gathered to date reinforced the credibility of the allegations concerning the transfer and temporary detention of individuals, without any judicial involvement, in European countries"
He said he deplored the fact that no information or explanations had been provided by US Secretary of State Condoleezza Rice, who faced repeated questions about the CIA prison allegations on her recent visit to Europe.
Marty has requested air traffic log books to try to determine flight patterns of several dozen suspected CIA aeroplanes.
He has also requested satellite pictures of the Sczytno-Szymany airport in northeastern Poland and the Mihail Kogalniceanu Air Base in eastern Romania, after they were identified by Human Rights Watch as possible sites of clandestine CIA detention centres.
After hearing Marty's presentation, a member of the Council of Europe committee, Tony Lloyd, said: "The really difficult thing is the idea that there is a kind of legal black hole in the middle of Europe."