"It would have been possible if the Italian government had had the courage to reach an agreement with the US government."
Prosecutors say the operation was a breach of Italian sovereignty that compromised Italian anti-terrorism efforts.
Armando Spataro, who has led the investigation, requested the charges last month in a case that highlights the CIA's alleged extraordinary rendition programme in which "terror" suspects are transferred to third countries where critics say they may face torture.
All but one of the Americans have been identified by the prosecution as CIA agents, including former station chiefs in Rome and Milan, and the 26th as a US Air Force officer stationed at the time at Aviano air base near Venice.
The Italians include the former head of the Italian military intelligence, Nicolo Pollari.
The preliminary hearing is closed to the public and is expected to continue over several days.
Defendants are not required to attend preliminary hearings, or even trials, in the Italian system.
Spataro has asked the government of Romano Prodi to seek the extradition of the American suspects, but there has not yet been a response.
The previous government of Silvio Berlusconi, who was a close ally of George Bush, the US president, refused.
Spataro has entered into evidence a hand-written letter allegedly smuggled out of an Egyptian prison
Osama Moustafa Hassan Nasr's letter describes his alleged abduction from a street in Milan in February 2003, and his journey, blindfolded and bound, to Egypt, where he said he had been left in an underground cell, repeatedly tortured and threatened with rape.
Prosecutors say Nasr was flown via the joint US-Italian Aviano air base first to a base in Germany and then on to Egypt.
The CIA has refused to comment on the case.
Nasr, an Egyptian cleric and "terror" suspect also known as Abu Omar, had been granted political asylum in Italy in 2001.
But he was also under investigation there for suspicion of involvement in acts of violence at the time of his disappearance, but had not been charged. He is currently being held at a police station in Alexandria, Egypt.