Switzerland is investigating whether foreign intelligence services acted illegally after reports its airspace was used by the CIA to transport suspected Islamic radicals to prisons outside the United States.
Federal prosecutors began investigations three weeks ago after the reports to see whether the intelligence services had committed illegal actions in Switzerland, a spokesman for the prosecution told Swiss television.
The investigations were directed against an unknown suspect, not the CIA specifically, he added.
"The question is not whether (representatives of) the foreign intelligence services were actually inside Switzerland. The question is whether they acted illegally," spokesman Hansjuerg Mark Wiedmer told Swiss television.
A European human rights investigator said on Tuesday in Paris that a month-long investigation had reinforced allegations the CIA ran a network of secret prisons in Europe, abducted prisoners and transferred them between countries.
Swiss senator Dick Marty, who is looking into the scandal for the 46-nation Council of Europe human rights watchdog, also criticised the United States for failing to come clean over the allegations.
The European Union and at least eight of its member states said last month they were seeking answers from the United States over the use of bases on the continent for secret prisoner transfers, known as "renditions".
Mr Wiedmer said there was little chance the investigations in Switzerland would lead to a lawsuit.
But, he said, it was important to find out what exactly had happened to see whether Switzerland's sovereignty had been violated.