The government ordered the destruction of documents on an alleged international nuclear smuggling network involving three Swiss engineers, it has been confirmed.
The head of a parliamentary control committee said the material was shredded last November.
The father and sons – Friedrich, Marco and Urs Tinner - are suspected of helping to supply parts for Libya's nuclear weapons programme between 2001 and 2003 through a trafficking ring run by Abdul Qadeer Khan, the father of Pakistan's atom bomb.
Reports say the three worked as undercover agents for the United States intelligence service.
There is widespread media speculation that Washington asked the Swiss government to destroy any evidence suggesting cooperation between the engineers and the CIA.
They were arrested nearly four years ago. One of them is still detained in Switzerland pending the outcome of a criminal investigation.
Experts say the destruction of the documents could hamper the probe and reduce the defence's case.