Speculation is rife whether an alleged Swiss spy arrested in Germany three weeks ago was acting on behalf of the Swiss authorities and whether they inadvertently blew his cover.
A watchdog rejected allegations that the Office of the Attorney General (OAG) was involved in spying activities of the suspect for the Federal Intelligence Service (FIS) or that it failed to black out passages in prosecution files, giving away his identity.
In a statement on Wednesday, the OAG watchdog urged other supervisory bodies to cooperate.
The 54-year-old former policeman and security consultant has also been under investigation in Switzerland since 2015 on suspicion of dealing in stolen banking data through German middlemen.
Reports say the interrogation records fell into the hands of German prosecutors investigating two German suspects in that case.
The case of the alleged Swiss spy triggered outrage in Germany, notably in the state of North Rhine-Westphalia which has bought at least 11 CD with stolen bank client data. It helped recover nearly €7 billion ($7.7 billion) in tax revenue.
The Neue Zürcher Zeitung newspaper on Wednesday quotes unnamed sources raising doubts about the reliability of the information provided by the lawyer of the suspect and a parliamentarian familiar with the case.
It remains unclear whether the suspect ever had a mandate by the FIS to spy on German tax inspectors trying to chase after German tax dodgers with Swiss bank accounts.
The Swiss government has remained tightlipped about the case ignoring calls for more transparency.
Last week, the Swiss government confirmed that the justice authorities asked the Federal Intelligence Service (FIS) to help with a case of stolen banking data which was sold to Germany.
The FIS ended its investigations in May 2014 which took place with the knowledge of the Swiss government, the cabinet spokesman said. He implicitly denied allegations that the case had damaged relations between Switzerland and neighbouring Germany.
The Green Party has called for a formal parliamentary investigation into the case. Several parliamentary committees are currently considering the case as part of their regular supervisory roles.
swissinfo.ch and agencies/urs