European MPs have flown to Macedonia to investigate claims by a German citizen that he was seized by US agents and taken to Afghanistan for interrogation.
Khaled al-Masri is suing the CIA over his detention
The MEPs are members of a committee probing allegations about secret CIA flights and prisons in Europe.
On Wednesday, MEPs said the CIA had run more than 1,000 flights within the EU since 2001, often transporting terror suspects for questioning overseas.
The US admits to some of the flights but denies condoning torture.
The MEPs are in Macedonia until Friday in order to investigate the case of Khaled al-Masri, a German citizen of Lebanese origin.
He was arrested in Macedonia in 2003. After that, what happened to him is not clear.
A report by MEPs says the CIA ran 1,000 flights within the EU
He says he was handed over to the Americans and taken to Afghanistan, where he was interrogated and mistreated.
After several months he was taken to Albania and released without charge, he says.
American officials did later acknowledge Mr Masri was mistaken for someone else and had been wrongly detained.
He is now suing the CIA and the Bavarian state prosecutor is investigating his claims.
The MEPs will meet the Macedonian prime minister and other senior officials.
However, the BBC's Alix Kroeger in Brussels says Macedonia is balancing its interests.
It has begun applying for EU membership but it is also keen to show itself to be an ally of the US in the war on terror, she says.
A European parliament report said many EU states had ignored the hundreds of CIA flights that had used their airports.
The report's author, Italian Socialist MEP Claudio Fava, singled out Sweden, Italy and Bosnia, which is not an EU member, for particular criticism.
A string of former detainees have come forward with stories alleging kidnap and transport by the US for interrogation in third countries - a process known as "extraordinary rendition".
Some have provided detailed accounts of alleged torture carried out in secret prisons outside EU or US jurisdiction.
Mr Fava accused European governments of breaching the Chicago aviation convention, under which all flights used for police purposes have to declare their route, destination, the names of crew and passengers.
None had been asked to do so by any European government.
Moreover, the flight paths, confirmed by the European air safety organisation Eurocontrol, "seem rather bizarre", he told the BBC.
According to Eurocontrol, the Boeing plane used for the abduction of Mr Masri flew on another occasion, between September 22-23, 2003, from Kabul to Poland to Romania to Morocco and to the US detention centre at Guantanamo Bay, Mr Fava said.
"It is hard to think that those stopovers were used simply to refuel," he said.
The committee has until next January to complete its investigation.
The BBC's Oana Lungescu in Brussels says the focus of the investigation will now shift to the possible existence of secret detention centres in Europe.
The European Parliament has uncovered no evidence so far, and neither has the Council of Europe, the human rights body also investigating the allegations.
Last year, Human Rights Watch said such centres were based in Poland and Romania - but both countries have strenuously denied any involvement, as have all the other European governments.