Suspects include 26 Americans, most believed to be CIA agents, as well as six Italians, including Nicolo Pollari, the former head of Italy's SISMI military intelligence agency.
Prosecutors believe the CIA agents, with help from SISMI, grabbed Hassan Mustafa Osama Nasr off a Milan street, bundled him into a van and flew him out of Italy from a US airbase.
Nasr, an Egyptian also known as Abu Omar, says he was tortured by Egyptian agents under questioning there with electric shocks, beatings, rape threats and genital abuse.
Nasr also said in an 11-page handwritten account that he was offered freedom if he collaborated with authorities, but he refused.
A high-ranking SISMI suspect says the CIA wanted the agency to help it abduct the imam but that he declined. The US embassy in Rome did not comment.
Washington acknowledges secret transfers of terrorism suspects to third countries, but denies torturing suspects or handing them to countries that do.
But a draft report issued last week by the European parliament said Nasr had been "held incommunicado and tortured" after his suspected abduction by the CIA and SISMI.
For the American suspects, who already face European Union arrest warrants, a trial would almost certainly take place in absentia since Washington is not expected to hand them over.
"I don't think he will come. There's an arrest warrant [for the Americans] and it's not over," said Daria Pesce, the lawyer chosen by Robert Lady, the former CIA station chief in Milan.
Lady is retired and lives in the US.
One suspect, an Italian police officer, has admitted he stopped Nasr and helped CIA agents grab him. But he says Lady told him the goal was to recruit - not abduct - the Muslim cleric. He also says he was told the US and Italian governments sanctioned the operation.
Italian police had been monitoring Nasr at the time of his abduction and say the so-called rendition ruined a promising investigation into terrorist networks in Europe.