The Swiss politician leading an investigation into allegations of secret CIA prisons says satellite images could reveal if prisoners were flown to eastern Europe.
Dick Marty, a member of the Senate, is trying to establish the facts for the Council of Europe, a human rights watchdog, which is looking into allegations published in the United States media.
In his position as chief investigator for the council, Marty in particular wants to investigate suspicious movements by 31 aircraft allegedly belonging to bodies with links to the Central Intelligence Agency.
They are suspected of having been used to transport prisoners in the "war against terrorism", according to a statement issued by Marty on Friday.
The Swiss, who hopes to present his full report to the council's parliamentary assembly in January, said he had contacted the European Union Satellite Centre based in Torrejon de Ardoz in Spain.
"With the help of precise geographic coordinates which I have obtained, it would be possible to obtain high-definition satellite images taken between the beginning of 2002 and now," he said.
The US-based Human Rights Watch organisation has identified Romania and European Union member Poland as countries that may have been used by the CIA in the alleged secret operation. Both countries have denied the charge.
Marty said Human Rights Watch had given him specific locations that might have been used as detention centres in those countries.
He also said he intended to ask Eurocontrol, the European Organisation for the Safety of Air Navigation, to follow up allegations that prisoners illegally detained by the US secret service might have transited through 31 European airports.
In a declaration adopted on Thursday at the council's parliamentary assembly in Strasbourg, it was stressed that the purpose of the inquiry was to establish the truth, not to "accuse" or to "sanction" one or more countries.
It added that a clear and strong message had to be sent that illegal and inhuman practices... could not be tolerated in any of the council's member states.
The Council's 46 members include both Poland and Romania.
Poland's Prime Minister Kazimierz Marcinkiewicz said on Thursday allegations that Polish airports had been used by CIA flights would be investigated.
Romania says it will allow investigations at two military bases to show they were not used.
In a related development, Spain said on Thursday it had sought and obtained the guarantee of the US authorities that, as far as they knew, there had been no violation of Spanish laws.
However, it added the government would step up checks on civilian plans that flew over or stopped in Spanish territory to ensure they were civilian flights.
swissinfo with agencies
Dick Marty was born in 1945 in Sorengo, canton Ticino.
He has a doctorate in law and works as a legal and economic consultant.
Since 1995, he has been a member of the Senate, representing the centre right Liberal-Radical Party.
He is a member of a number of commissions, including foreign policy, economy and legal affairs.
He also has a seat in the Council of Europe's parliamentary assembly. Earlier this month he became chairman of the council's legal-affairs committee.
The Council of Europe is Europe's oldest political organisation, founded in 1949.
It comprises 46 countries, including 21 from eastern and central Europe.
The Strasbourg-based Council is independent from the 25-nation European Union.
The Council defends human rights and parliamentary democracy.